fix(te-plugin): make _Linear arg indexing robust to TE signature changes (#1473)

### What does this PR do?

Type of change: Bug fix

ModelOpt's `te_quantized_linear_fn` and `te_grouped_quantized_linear_fn`
read `weight` / `inp` from hard-coded positions in `args`. Two TE
signature changes broke this scheme:

- **TE 1.x → 2.0:** dropped the legacy `weight_fp8` slot between
`weight` and `inp`. ModelOpt handled this with an `if Version("2.0") <=
_TE_VERSION:` branch + a duplicate else branch.
- **TE 2.14 → 2.15:** inserted `weight_workspace` between `weight` and
`inp` at the `_Linear.forward` call site ([TE 2.15 linear.py
L1663](https://github.com/NVIDIA/TransformerEngine/blob/release_v2.15/transformer_engine/pytorch/module/linear.py#L1663)).
Unhandled by ModelOpt — `args[idx + 1]` resolved to `None` (workspace is
None outside FP8), which then crashed `TensorQuantizer.forward` on
`inputs.numel()` with `AttributeError: 'NoneType' object has no
attribute 'numel'`. Surfaced as a regression in Megatron-Bridge after
the TE 2.15 bump alongside ModelOpt 0.44.0rc3.
- **TE 2.10:** `_GroupedLinear.forward`'s second positional slot was
renamed `m_splits` → `non_tensor_args` (tuple wrapping). ModelOpt had a
separate `Version("2.10")` gate for this.

Replace all three version gates with **parameter-name introspection** of
the live `_Linear.forward` / `_GroupedLinear.forward` signature. The
parameter names (`weight`, `inp`, `m_splits`, `non_tensor_args`) have
been stable across TE 1.x, 2.x, and 2.15+; only their relative positions
shift. The new code reads the live signature via
`inspect.signature(...).parameters`, locates `weight`/`inp` by name, and
mutates only those positions in a list copy of `args` — everything
between (e.g. TE 2.15's `weight_workspace`) and after passes through
verbatim. The dual-branch code in `te_quantized_linear_fn` collapses to
a single path.

### Usage

No public API change. PTQ continues to work transparently across all
supported TE versions:

```python
import modelopt.torch.quantization as mtq
# Works on TE 1.x, 2.0-2.14, 2.15.x, and 2.16+ — no version flag needed.
mtq.quantize(model, mtq.NVFP4_DEFAULT_CFG, forward_loop)
```

### Testing
<!-- Mention how have you tested your change if applicable. -->

Existing TE plugin tests
(`tests/gpu_megatron/torch/quantization/plugins/test_transformer_engine.py`)
exercise both the `_forward` (no-grad calibration) and `_apply`
(grad-enabled training) paths of `te_quantized_linear_fn` for
`te.pytorch.Linear` — they would have caught the original TE 2.15
regression on a CI matrix entry pinned to TE 2.15. Verified trace
correctness across:

| TE version | `_Linear.forward` signature | `_te_linear` weight→inp gap
| `_GroupedLinear.forward` second slot |
|---|---|---|---|
| 1.x | `(ctx, weight, weight_fp8, inp, …)` | 1 | n/a |
| 2.0–2.14 | `(ctx, weight, inp, bias, …)` | 0 | `m_splits` |
| 2.15.x | `(ctx, weight, weight_workspace, inp, …)` | 1 |
`non_tensor_args` |
| 2.16+ (main) | `(ctx, weight, inp, bias, fwd_args)` | 0 |
`non_tensor_args` |

### Before your PR is "*Ready for review*"

Make sure you read and follow [Contributor
guidelines](https://github.com/NVIDIA/Model-Optimizer/blob/main/CONTRIBUTING.md)
and your commits are signed (`git commit -s -S`).

Make sure you read and follow the [Security Best
Practices](https://github.com/NVIDIA/Model-Optimizer/blob/main/SECURITY.md#security-coding-practices-for-contributors)
(e.g. avoiding hardcoded `trust_remote_code=True`, `torch.load(...,
weights_only=False)`, `pickle`, etc.).

- Is this change backward compatible?: ✅ <!--- Public API unchanged;
broadens the range of TE versions that work (TE 2.15.x now supported, TE
1.x still supported via the same introspection path). -->
- If you copied code from any other sources or added a new PIP
dependency, did you follow guidance in `CONTRIBUTING.md`: N/A <!--- Only
adds a stdlib `inspect` import. -->
- Did you write any new necessary tests?: Existing tests sufficient
<!--- Bug fix is covered by existing `test_transformer_engine.py` for
whatever single TE version CI exercises. A multi-version TE matrix is
the right next step but is out of scope for this PR. -->

### Additional Information
<!-- E.g. related issue. -->
Triggered by Megatron-Bridge
NVIDIA-NeMo/Megatron-Bridge#3783 failing tests
after bumping ModelOpt 0.44.0rc2 → 0.44.0rc3 together with a Megatron-LM
bump that pulls TE 2.15. ModelOpt rc2 had the same latent bug — it just
wasn't exercised until TE 2.15 became the runtime version.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Refactor**
* Improved Transformer Engine quantization plugin robustness by using
runtime parameter inspection instead of version-based branching,
ensuring compatibility across TE versions without requiring manual
updates.

[![Review Change
Stack](https://storage.googleapis.com/coderabbit_public_assets/review-stack-in-coderabbit-ui.svg)](https://app.coderabbit.ai/change-stack/NVIDIA/Model-Optimizer/pull/1473)

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Signed-off-by: Keval Morabia <28916987+kevalmorabia97@users.noreply.github.com>

Author: kevalmorabia97

modelopt/torch/quantization/plugins/transformer_engine.py

@@ -15,6 +15,7 @@
 
 """Support quantization for Transformer Engine layers."""
 
+import inspect
 import warnings
 
 import torch
@@ -74,30 +75,24 @@ def _setup(self):
     def te_quantized_linear_fn(package, func_name, self, *args, **kwargs):
         """Quantized version specifically for TE with weight first, then input."""
         _assert_te_fp8_enabled()
-        if Version("2.0") <= _TE_VERSION:
-            idx = 1 if func_name == "_forward" else 0
-            weight, inputs = args[idx], args[idx + 1]
-            remaining_args = args[idx + 2 :]
-            weight = self.weight_quantizer(weight)
-            inputs = self.input_quantizer(inputs)
-            new_args = (weight, inputs, *remaining_args)
-            new_args = (args[0], *new_args) if func_name == "_forward" else new_args
-            output = getattr(package, func_name)(
-                *new_args,
-                **kwargs,
-            )
-        else:
-            idx = 1 if func_name == "_forward" else 0
-            weight, weight_fp8, inputs = args[idx], args[idx + 1], args[idx + 2]
-            remaining_args = args[idx + 3 :]
-            weight = self.weight_quantizer(weight)
-            inputs = self.input_quantizer(inputs)
-            new_args = (weight, weight_fp8, inputs, *remaining_args)
-            new_args = (args[0], *new_args) if func_name == "_forward" else new_args
-            output = getattr(package, func_name)(
-                *new_args,
-                **kwargs,
-            )
+        # Locate `weight` and `inp` by parameter name in the un-patched `_Linear.forward`
+        # signature — robust to TE versions that insert positional args between them
+        # (e.g. `weight_fp8` in TE 1.x, `weight_workspace` in TE 2.15).
+        # NOTE: we're called from inside `replace_function`'s context, so
+        # `_Linear.forward` may currently point at the `functools.partial` wrapper
+        # (whose signature collapses to `*args, **kwargs`). The original is cached at
+        # `_Linear._forward` while the patch is active (when `_apply` is patched
+        # instead, `_forward` is absent and `forward` is itself the original).
+        # `_forward` path receives a leading None (placeholder ctx); `_apply` does not.
+        orig_forward = getattr(te_linear._Linear, "_forward", te_linear._Linear.forward)
+        names = list(inspect.signature(orig_forward).parameters)
+        ctx_offset = 0 if func_name == "_forward" else 1
+        weight_pos = names.index("weight") - ctx_offset
+        inp_pos = names.index("inp") - ctx_offset
+        new_args = list(args)
+        new_args[weight_pos] = self.weight_quantizer(args[weight_pos])
+        new_args[inp_pos] = self.input_quantizer(args[inp_pos])
+        output = getattr(package, func_name)(*new_args, **kwargs)
         return self.output_quantizer(output)
 
     # Override the quantized linear function
@@ -161,35 +156,31 @@ def iter_weights_for_calibration(self):
     @staticmethod
     def te_grouped_quantized_linear_fn(package, func_name, self, *args):
         _assert_te_fp8_enabled()
-        idx = 1 if func_name == "_forward" else 0
-        inp = args[idx]
-
-        # Handle both old and new TE signatures (changed in PR #2377 in TE 2.10)
-        # New signature (TE >= 2.10): forward(ctx, inp, non_tensor_args: Tuple, *weights_and_biases)
-        # Old signature (TE < 2.10): forward(ctx, inp, m_splits: List[int], use_bias, ...)
-        if Version("2.10") <= _TE_VERSION:
-            # New signature: non_tensor_args is a tuple, m_splits is the first element
-            num_gemms = len(args[idx + 1][0])
-        else:
-            # Old signature: m_splits is directly args[idx + 1]
-            num_gemms = len(args[idx + 1])
-
-        weights_and_biases = args[-2 * num_gemms :]
-        weights, biases = weights_and_biases[:num_gemms], weights_and_biases[num_gemms:]
-        quantized_inputs = self.input_quantizer(inp)
-        quantized_weights = [self.weight_quantizer(weight) for weight in weights]
-
-        output = getattr(package, func_name)(
-            *(
-                args[0],
-                quantized_inputs,
-            )
-            if func_name == "_forward"
-            else (quantized_inputs,),
-            *args[idx + 1 : -2 * num_gemms],
-            *quantized_weights,
-            *biases,
+        # Locate `inp` and the m_splits-bearing arg by parameter name. The second
+        # slot was renamed from `m_splits` (TE < 2.10) to `non_tensor_args` (TE
+        # 2.10+, where m_splits is now at non_tensor_args[0]). `*weights_and_biases`
+        # is always the trailing variadic — 2 * num_gemms tensors (weights, then biases).
+        # See `te_quantized_linear_fn` for why we look up `_forward` here.
+        # `_forward` path receives a leading None (placeholder ctx); `_apply` does not.
+        orig_forward = getattr(
+            te_grouped_linear._GroupedLinear,
+            "_forward",
+            te_grouped_linear._GroupedLinear.forward,
         )
+        sig_params = list(inspect.signature(orig_forward).parameters)
+        ctx_offset = 0 if func_name == "_forward" else 1
+        inp_pos = sig_params.index("inp") - ctx_offset
+        if "non_tensor_args" in sig_params:
+            num_gemms = len(args[sig_params.index("non_tensor_args") - ctx_offset][0])
+        else:
+            num_gemms = len(args[sig_params.index("m_splits") - ctx_offset])
+        weights_start = len(args) - 2 * num_gemms
+
+        new_args = list(args)
+        new_args[inp_pos] = self.input_quantizer(args[inp_pos])
+        for i in range(weights_start, weights_start + num_gemms):
+            new_args[i] = self.weight_quantizer(args[i])
+        output = getattr(package, func_name)(*new_args)
         return self.output_quantizer(output)
 
     # Override the quantized linear function