[Relax][Frontend][TFLite] Add UNIDIRECTIONAL_SEQUENCE_RNN converter (#19601)

## Summary

This PR adds Relax TFLite frontend support for
`UNIDIRECTIONAL_SEQUENCE_RNN` (BuiltinOperator 35), claimed in
[#19519](#19519) Group A.

The op executes a simple RNN cell over a time sequence. The converter
unrolls the time steps at graph-construction time using Relax
primitives.

Cell equation:
```
h_t = fused_activation(x_t @ W.T + h_{t-1} @ Wr.T + b)
```

## Changes

- **Handler**: `convert_unidirectional_sequence_rnn` registered in
`convert_map` (alphabetical, U-region after `UNPACK`)
- **Inputs** (5): `input [batch, time, input_size]`, `input_weights
[num_units, input_size]`, `recurrent_weights [num_units, num_units]`,
`bias [num_units]`, `hidden_state [batch, num_units]` (variable,
zero-initialised)
- **Output**: `[batch, time, num_units]` (always batch-major)
- **time_major=True**: input is transposed to batch-major before
unrolling
- **Activations**: NONE, RELU, RELU6, TANH, SIGMOID (via
`convert_fused_activation_function`)
- **Quantized**: raises `OpNotImplemented` (not yet supported)

## Testing

Modern TF/Keras (2.x, Keras 3) no longer emits
`UNIDIRECTIONAL_SEQUENCE_RNN`; `SimpleRNN` with `unroll=False` lowers to
`WHILE`+TensorList ops, and `unroll=True` expands to elementwise ops.
Tests therefore follow the same flatbuffer-construction pattern used by
the StableHLO op PRs (#19536, #19587).

Three tests added to `tests/python/relax/test_frontend_tflite.py`:

- `test_unidirectional_sequence_rnn_none_activation` —
`tvm.ir.assert_structural_equal` with identity weights / zero bias, NONE
activation, time=1
- `test_unidirectional_sequence_rnn_relu_activation` — shape check,
random weights, RELU activation, time=3
- `test_unidirectional_sequence_rnn_time_major` — shape check,
`time_major=True` input layout

```bash
python -m pytest tests/python/relax/test_frontend_tflite.py -k unidirectional_sequence_rnn -v
```

All 3 tests pass. pre-commit (ASF header, ruff check, ruff format) all
pass.

## References

- Issue [#19519](#19519) Group A:
Sequence / recurrent model operators

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: LudovicoYIN

python/tvm/relax/frontend/tflite/tflite_frontend.py

@@ -381,6 +381,7 @@ def __init__(self, model, subgraph, exp_tab, ctx, conversion_state=None):
             "TRANSPOSE_CONV": self.convert_transpose_conv,
             "TRANSPOSE": self.convert_transpose,
             "UNPACK": self.convert_unpack,
+            "UNIDIRECTIONAL_SEQUENCE_RNN": self.convert_unidirectional_sequence_rnn,
             "UNSORTED_SEGMENT_MIN": functools.partial(
                 self._convert_segment_op, op_name="UNSORTED_SEGMENT_MIN", reduction="min"
             ),
@@ -4877,6 +4878,106 @@ def convert_unpack(self, op):
 
         return squeezed
 
+    def convert_unidirectional_sequence_rnn(self, op):
+        """Convert TFLite UNIDIRECTIONAL_SEQUENCE_RNN.
+
+        Inputs (5 tensors):
+          [0] input          [batch, time, input_size]  (or [time, batch, input_size] if time_major)
+          [1] input_weights  [num_units, input_size]
+          [2] recurrent_weights [num_units, num_units]
+          [3] bias           [num_units]
+          [4] hidden_state   [batch, num_units]  (variable, zero-initialised)
+
+        Output:
+          [0] output  [batch, time, num_units]
+
+        Cell equation:
+          h_t = fused_activation(x_t @ W.T + h_{t-1} @ Wr.T + b)
+        """
+        from tflite.BuiltinOptions import BuiltinOptions
+        from tflite.SequenceRNNOptions import SequenceRNNOptions
+
+        if self.is_quantized(op):
+            raise tvm.error.OpNotImplemented(
+                "TFLite quantized UNIDIRECTIONAL_SEQUENCE_RNN is not supported yet."
+            )
+
+        input_tensors = self.get_input_tensors(op)
+        assert len(input_tensors) == 5, "input tensors length should be 5"
+
+        input_tensor = input_tensors[0]
+        weights_tensor = input_tensors[1]
+        recurrent_tensor = input_tensors[2]
+        bias_tensor = input_tensors[3]
+        hidden_state_tensor = input_tensors[4]
+
+        output_tensors = self.get_output_tensors(op)
+        assert len(output_tensors) >= 1, "output tensors length should be at least 1"
+
+        assert op.BuiltinOptionsType() == BuiltinOptions.SequenceRNNOptions
+        op_options = op.BuiltinOptions()
+        seq_rnn_options = SequenceRNNOptions()
+        seq_rnn_options.Init(op_options.Bytes, op_options.Pos)
+        time_major = seq_rnn_options.TimeMajor()
+        fused_activation_fn = seq_rnn_options.FusedActivationFunction()
+
+        # Constant weight/bias expressions.
+        weights_expr = self.get_tensor_expr(weights_tensor)  # [num_units, input_size]
+        recurrent_expr = self.get_tensor_expr(recurrent_tensor)  # [num_units, num_units]
+
+        # bias is optional (tensor_idx == -1 when absent); default to zeros.
+        if bias_tensor.tensor_idx != -1:
+            bias_expr = self.get_tensor_expr(bias_tensor)  # [num_units]
+        else:
+            num_units = int(self.get_tensor_shape(weights_tensor)[0])
+            bias_dtype = self.get_tensor_type_str(weights_tensor.tensor.Type())
+            bias_expr = relax.op.zeros((num_units,), dtype=bias_dtype)
+
+        # Transpose to [input_size, num_units] and [num_units, num_units] for x @ W.T.
+        w_t = relax.op.permute_dims(weights_expr)
+        wr_t = relax.op.permute_dims(recurrent_expr)
+
+        # Resolve the input expression; normalise to batch-major [batch, time, input_size].
+        # Only the time dimension must be static (needed for unrolling); batch may be dynamic.
+        in_expr = self.get_tensor_expr(input_tensor)
+        in_shape = self.get_tensor_shape(input_tensor)
+        if time_major:
+            in_expr = relax.op.permute_dims(in_expr, [1, 0, 2])
+            num_steps = int(in_shape[0])
+        else:
+            num_steps = int(in_shape[1])
+
+        # Initial hidden state: use the model's tensor value when available (non-zero init or
+        # graph input), otherwise fall back to zeros for the common variable-tensor case.
+        h_dtype = self.get_tensor_type_str(hidden_state_tensor.tensor.Type())
+        if self.has_expr(hidden_state_tensor.tensor_idx) or (
+            hidden_state_tensor.buffer is not None and hidden_state_tensor.buffer.DataLength() > 0
+        ):
+            h = self.get_tensor_expr(hidden_state_tensor)
+        else:
+            h_shape = tuple(to_int_list(self.get_tensor_shape(hidden_state_tensor)))
+            h = relax.op.zeros(h_shape, dtype=h_dtype)
+
+        # Unroll over the time axis.
+        # relax.op.split with 1 section returns the tensor directly; handle uniformly.
+        if num_steps == 1:
+            steps = [relax.op.squeeze(in_expr, axis=[1])]
+        else:
+            splits = relax.op.split(in_expr, num_steps, axis=1)
+            steps = [relax.op.squeeze(splits[i], axis=[1]) for i in range(num_steps)]
+
+        outputs = []
+        for x_t in steps:  # x_t: [batch, input_size]
+            gates = relax.op.add(
+                relax.op.add(relax.op.matmul(x_t, w_t), relax.op.matmul(h, wr_t)),
+                bias_expr,
+            )
+            h = self.convert_fused_activation_function(gates, fused_activation_fn)
+            outputs.append(h)
+
+        # Stack timestep outputs: [batch, time, num_units].
+        return relax.op.stack(outputs, axis=1)
+
     """
     def convert_unidirectional_sequence_lstm(self, op):
         ### Long Short Term Memory for TFLite implementation. ###

tests/python/relax/test_frontend_tflite.py

@@ -3720,6 +3720,8 @@ def _get_tflite_schema_enum(enum_name):
 _tfl_sparse_index_vector = _get_tflite_schema_enum("SparseIndexVector")
 _tfl_tensor_type = _get_tflite_schema_enum("TensorType")
 
+_tfl_sequence_rnn_options = _get_tflite_schema_module("SequenceRNNOptions")
+
 _DENSIFY_TEST_VALUES = np.array([1.0, 2.0], dtype=np.float32)
 _DENSIFY_TEST_DENSE = np.array([[1.0, 0.0], [0.0, 2.0]], dtype=np.float32)
 _DENSIFY_ROW_PTRS = [0, 1, 2]
@@ -9719,5 +9721,210 @@ def main(
     tvm.ir.assert_structural_equal(mod, Expected)
 
 
+# ── UNIDIRECTIONAL_SEQUENCE_RNN ───────────────────────────────────────────────
+
+
+def _build_unidirectional_sequence_rnn_model(
+    batch,
+    time,
+    input_size,
+    num_units,
+    weights,
+    recurrent_weights,
+    bias,
+    activation,
+    *,
+    time_major=False,
+):
+    """Build a minimal TFLite flatbuffer model containing one UNIDIRECTIONAL_SEQUENCE_RNN op.
+
+    Tensor layout (indices 0-5):
+      0 - input          [batch, time, input_size]  (or [time, batch, input_size] if time_major)
+      1 - input_weights  [num_units, input_size]    (constant)
+      2 - recurrent_wts  [num_units, num_units]     (constant)
+      3 - bias           [num_units]                (constant)
+      4 - hidden_state   [batch, num_units]         (variable, zero-initialised)
+      5 - output         [batch, time, num_units]
+    """
+    builder = flatbuffers.Builder(4096)
+
+    _tfl_sequence_rnn_options.SequenceRNNOptionsStart(builder)
+    _tfl_sequence_rnn_options.SequenceRNNOptionsAddTimeMajor(builder, time_major)
+    _tfl_sequence_rnn_options.SequenceRNNOptionsAddFusedActivationFunction(builder, activation)
+    rnn_opts = _tfl_sequence_rnn_options.SequenceRNNOptionsEnd(builder)
+
+    rnn_op_code = _build_operator_code(builder, _tfl_builtin_operator.UNIDIRECTIONAL_SEQUENCE_RNN)
+
+    input_shape = [time, batch, input_size] if time_major else [batch, time, input_size]
+
+    def _t(buf_idx, shape, is_variable=False):
+        shape_vec = _tflite_shape(builder, shape)
+        _tfl_tensor.TensorStart(builder)
+        _tfl_tensor.TensorAddBuffer(builder, buf_idx)
+        _tfl_tensor.TensorAddHasRank(builder, True)
+        _tfl_tensor.TensorAddIsVariable(builder, is_variable)
+        _tfl_tensor.TensorAddShape(builder, shape_vec)
+        _tfl_tensor.TensorAddType(builder, _tfl_tensor_type.FLOAT32)
+        return _tfl_tensor.TensorEnd(builder)
+
+    tensors = [
+        _t(0, input_shape),
+        _t(1, [num_units, input_size]),
+        _t(2, [num_units, num_units]),
+        _t(3, [num_units]),
+        _t(4, [batch, num_units], is_variable=True),
+        _t(5, [batch, time, num_units]),
+    ]
+
+    rnn_op = _build_operator(
+        builder,
+        0,
+        [0, 1, 2, 3, 4],
+        [5],
+        builtin_options_type=_tfl_builtin_options.SequenceRNNOptions,
+        builtin_options=rnn_opts,
+    )
+
+    subgraph = _build_subgraph(
+        builder,
+        tensors=tensors,
+        operators=[rnn_op],
+        inputs=[0],
+        outputs=[5],
+    )
+
+    buffers = [
+        _build_buffer(builder),
+        _build_buffer(builder, weights.tobytes()),
+        _build_buffer(builder, recurrent_weights.tobytes()),
+        _build_buffer(builder, bias.tobytes()),
+        _build_buffer(builder),
+        _build_buffer(builder),
+    ]
+
+    return _finish_tflite_model(
+        builder,
+        subgraph=subgraph,
+        operator_codes=[rnn_op_code],
+        buffers=buffers,
+    )
+
+
+def test_unidirectional_sequence_rnn_none_activation():
+    """UNIDIRECTIONAL_SEQUENCE_RNN with NONE activation, time=1, lowers to matmul/add/stack.
+
+    Cell equation: h_t = x_t @ W.T + h_{t-1} @ Wr.T + b  (no activation for NONE)
+    """
+    from tflite.ActivationFunctionType import ActivationFunctionType
+
+    batch, time, input_size, num_units = 2, 1, 2, 2
+    weights = np.eye(num_units, input_size, dtype=np.float32)
+    recurrent_weights = np.eye(num_units, dtype=np.float32)
+    bias = np.zeros(num_units, dtype=np.float32)
+
+    mod = _load_model_from_buffer(
+        _build_unidirectional_sequence_rnn_model(
+            batch,
+            time,
+            input_size,
+            num_units,
+            weights,
+            recurrent_weights,
+            bias,
+            ActivationFunctionType.NONE,
+        )
+    )
+
+    @I.ir_module
+    class Expected:
+        @R.function
+        def main(x: R.Tensor((2, 1, 2), dtype="float32")) -> R.Tensor((2, 1, 2), dtype="float32"):
+            R.func_attr({"num_input": 1})
+            with R.dataflow():
+                lv: R.Tensor((2, 2), dtype="float32") = R.squeeze(x, axis=[1])
+                lv1: R.Tensor((2, 2), dtype="float32") = R.permute_dims(
+                    R.const(np.eye(2, dtype=np.float32)), axes=None
+                )
+                lv2: R.Tensor((2, 2), dtype="float32") = R.matmul(lv, lv1, out_dtype="void")
+                lv3: R.Tensor((2, 2), dtype="float32") = R.zeros(R.shape([2, 2]), dtype="float32")
+                lv4: R.Tensor((2, 2), dtype="float32") = R.permute_dims(
+                    R.const(np.eye(2, dtype=np.float32)), axes=None
+                )
+                lv5: R.Tensor((2, 2), dtype="float32") = R.matmul(lv3, lv4, out_dtype="void")
+                lv6: R.Tensor((2, 2), dtype="float32") = R.add(lv2, lv5)
+                lv7: R.Tensor((2, 2), dtype="float32") = R.add(
+                    lv6, R.const(np.zeros(2, dtype=np.float32))
+                )
+                gv: R.Tensor((2, 1, 2), dtype="float32") = R.stack((lv7,), axis=1)
+                R.output(gv)
+            return gv
+
+    tvm.ir.assert_structural_equal(mod, Expected)
+
+
+def test_unidirectional_sequence_rnn_relu_activation():
+    """UNIDIRECTIONAL_SEQUENCE_RNN with RELU activation and multiple time steps."""
+    from tflite.ActivationFunctionType import ActivationFunctionType
+
+    batch, time, input_size, num_units = 2, 3, 4, 8
+    np.random.seed(42)
+    weights = np.random.randn(num_units, input_size).astype(np.float32)
+    recurrent_weights = np.random.randn(num_units, num_units).astype(np.float32)
+    bias = np.random.randn(num_units).astype(np.float32)
+
+    mod = _load_model_from_buffer(
+        _build_unidirectional_sequence_rnn_model(
+            batch,
+            time,
+            input_size,
+            num_units,
+            weights,
+            recurrent_weights,
+            bias,
+            ActivationFunctionType.RELU,
+        )
+    )
+
+    fn = mod["main"]
+    assert len(fn.params) == 1, "only the sequence input should be a graph input"
+    in_shape = fn.params[0].struct_info.shape
+    assert tuple(int(d) for d in in_shape) == (batch, time, input_size)
+    out_shape = fn.ret_struct_info.shape
+    assert tuple(int(d) for d in out_shape) == (batch, time, num_units)
+
+
+def test_unidirectional_sequence_rnn_time_major():
+    """UNIDIRECTIONAL_SEQUENCE_RNN with time_major=True transposes before unrolling."""
+    from tflite.ActivationFunctionType import ActivationFunctionType
+
+    batch, time, input_size, num_units = 3, 4, 2, 5
+    np.random.seed(7)
+    weights = np.random.randn(num_units, input_size).astype(np.float32)
+    recurrent_weights = np.random.randn(num_units, num_units).astype(np.float32)
+    bias = np.zeros(num_units, dtype=np.float32)
+
+    mod = _load_model_from_buffer(
+        _build_unidirectional_sequence_rnn_model(
+            batch,
+            time,
+            input_size,
+            num_units,
+            weights,
+            recurrent_weights,
+            bias,
+            ActivationFunctionType.NONE,
+            time_major=True,
+        )
+    )
+
+    fn = mod["main"]
+    # Input to the graph is the raw time-major tensor [time, batch, input_size].
+    in_shape = fn.params[0].struct_info.shape
+    assert tuple(int(d) for d in in_shape) == (time, batch, input_size)
+    # Output is always batch-major [batch, time, num_units].
+    out_shape = fn.ret_struct_info.shape
+    assert tuple(int(d) for d in out_shape) == (batch, time, num_units)
+
+
 if __name__ == "__main__":
     pytest.main(["-s", __file__])