[Relax][Frontend][TFLite] Support dynamic DYNAMIC_UPDATE_SLICE starts

[Aharrypotter]

Summary

This PR adds Relax TFLite frontend support for runtime (dynamic) start indices
in STABLEHLO_DYNAMIC_UPDATE_SLICE, addressing the DYNAMIC_UPDATE_SLICE item
from #19412 section B.

_convert_stablehlo_dynamic_update_slice (added in #19587) previously raised
OpNotImplemented when the start-index scalars were runtime (non-constant)
values, handling only compile-time-constant starts. Models that compute the
update offset at runtime could therefore not be imported. This PR makes the
dynamic-start path work, with StableHLO clamping semantics, without adding a new
Relax op. The change is limited to this converter and its test.

Design

Dynamic start indices via scatter_nd

The existing static path already lowers STABLEHLO_DYNAMIC_UPDATE_SLICE to
relax.op.scatter_nd, building the scatter index grid at compile time with
numpy.indices. scatter_nd accepts a general runtime indices tensor and
returns the data (operand) shape unchanged, so the dynamic case needs no new
op and introduces no symbolic dimensions — only the index grid is built
in-graph instead of in NumPy.

For runtime starts, the converter builds the index grid per axis a (rank is
statically known from the operand/update shapes):

• clamp the start to [0, operand_dim - update_dim] with relax.op.maximum /
  relax.op.minimum — StableHLO clamps out-of-range starts rather than erroring;
• idx = arange(update_dim) + clamped_start;
• reshape idx to broadcast on axis a and broadcast_to the update shape;
• expand_dims a trailing index axis.

concat over the axes produces an int64 index tensor of shape
(*update_shape, rank), which is fed to the same
relax.op.scatter_nd(operand, indices, update, "update") call the static path
uses.

The static (constant-start) path is unchanged, including its compile-time
out-of-bounds rejection.

Operator Support

Operator	TFLite inputs	Relax lowering	Supported subset
STABLEHLO_DYNAMIC_UPDATE_SLICE	operand, update, N scalar start indices	relax.op.scatter_nd with a NumPy index grid (constant starts) or an in-graph arange + clamp index grid (runtime starts)	static operand/update shapes; constant or runtime start indices

Not Included

• Dynamic (non-static) operand or update shapes — the index grid is built from
  the statically known update shape, so operand/update shapes must be static.
  Runtime start indices are supported; runtime tensor shapes are not.

Tests

The dynamic-start test compiles the imported module and runs it on the Relax VM,
comparing the output against a NumPy reference; it includes an out-of-range start
to exercise clamping. The static structural-equal and out-of-bounds tests are
unchanged.

Test	Coverage
test_stablehlo_dynamic_update_slice	constant start indices, structural-equal (existing)
test_stablehlo_dynamic_update_slice_dynamic_starts	runtime start indices, compile + run, including an out-of-range start that is clamped
test_stablehlo_dynamic_update_slice_out_of_bounds_unsupported	constant-start path rejects out-of-bounds updates (existing)

Local validation:

python -m ruff format --check \
  python/tvm/relax/frontend/tflite/tflite_frontend.py \
  tests/python/relax/test_frontend_tflite.py

python -m ruff check \
  python/tvm/relax/frontend/tflite/tflite_frontend.py \
  tests/python/relax/test_frontend_tflite.py

python -m pytest \
  tests/python/relax/test_frontend_tflite.py -k dynamic_update_slice -q

python -m pytest \
  tests/python/relax/test_frontend_tflite.py -q

Result:

ruff format --check: 2 files already formatted
ruff check: All checks passed
dynamic_update_slice tests: 3 passed, 555 deselected
full TFLite pytest: 558 passed

References

• Issue [Tracking Issue][TFlite] Expand Missing / Partial Operator Coverage #19412 section B: DYNAMIC_UPDATE_SLICE
• PR [Relax][Frontend][TFLite] Support StableHLO region-based ops and multi-subgraph models #19587: introduced STABLEHLO_DYNAMIC_UPDATE_SLICE (constant starts) and
  multi-subgraph / StableHLO region support

[gemini-code-assist]

Code Review

This pull request adds support for converting STABLEHLO_DYNAMIC_UPDATE_SLICE with dynamic start indices in the TVM Relax TFLite frontend by lowering it to relax.op.scatter_nd and building the index grid dynamically at runtime. It also adds a test case to verify correct execution and clamping behavior. Feedback suggests improving robustness by explicitly passing start, stop, and step constants to relax.op.arange to avoid type mismatches, and adding compile-time validation to ensure update slice dimensions do not exceed operand dimensions.

Important

The consumer version of Gemini Code Assist on GitHub is being sunset. Starting June 18, 2026, new organization installations will be blocked, and all code review activity will officially cease on July 17, 2026.
For more details on the timeline and next steps, please review the Help Documentation.

[tlopex]

Could you change the test to use tvm.ir.assert_structural_equal due to the CI pressure?
Also we'd better use structural check for frontend tests whenever possible in the future