🐛 CI failure: MultiTokenPredictionLayer._checkpointed_forward() got unexpected kwarg 'padding_mask'

**Describe the bug**

CI test `tests/unit_tests/transformer/test_multi_token_prediction.py::TestMultiTokenPrediction` is failing across many parametrizations with a `TypeError` on the recompute path:

```
megatron/core/transformer/multi_token_prediction.py:1301: TypeError
E TypeError: MultiTokenPredictionLayer._checkpointed_forward() got an unexpected keyword argument 'padding_mask'
```

Failing nodes (all in the same job):

- `TestMultiTokenPrediction::test_forward_backward[{1,2,4}-{1,2,4}-True]` (9 combinations)
- `TestMultiTokenPrediction::test_fp8_support[True]`
- `TestMultiTokenPrediction::test_packed_sequences_with_full_recompute`

Tag @NVIDIA/mcore-oncall to get oncall's attention to this issue.

**Root cause (likely)**

The call site at `megatron/core/transformer/multi_token_prediction.py:1301` passes `padding_mask=padding_mask` into `self._checkpointed_forward(...)`, but the method definition at `megatron/core/transformer/multi_token_prediction.py:1093` does not declare a `padding_mask` parameter.

`git blame` points to two recent landings that don't compose:

- `2d1fa8d372` ([#2645](https://github.com/NVIDIA/Megatron-LM/pull/2645), 2026-05-14, @Connor-XY) added `padding_mask=padding_mask` to the call site.
- `2b77d32b1e` ([#4593](https://github.com/NVIDIA/Megatron-LM/pull/4593), 2026-05-21, @BestJuly) refactored `_checkpointed_forward` without including `padding_mask` in the new signature.

The recompute branch (`config.recompute_granularity == 'full' and self.training`) crashes on every call.

**Failing run**

| Field | Value |
|-------|-------|
| PR | [#4931: test: enable NVTE_CUTEDSL_FUSED_GROUPED_MLP via pytest fixture](https://github.com/NVIDIA/Megatron-LM/pull/4931) (surfaced here; the PR itself does not touch MTP) |
| Run | [26290097610](https://github.com/NVIDIA/Megatron-LM/actions/runs/26290097610) |
| Job | [`tests/unit_tests/transformer/test_multi_token_prediction.py - latest`](https://github.com/NVIDIA/Megatron-LM/actions/runs/26290097610/job/77389398708) |

**Error (verbatim, abridged)**

```
megatron/core/transformer/multi_token_prediction.py:1301: TypeError
E TypeError: MultiTokenPredictionLayer._checkpointed_forward() got an unexpected keyword argument 'padding_mask'

FAILED tests/unit_tests/transformer/test_multi_token_prediction.py::TestMultiTokenPrediction::test_forward_backward[1-1-True]
FAILED tests/unit_tests/transformer/test_multi_token_prediction.py::TestMultiTokenPrediction::test_forward_backward[1-2-True]
FAILED tests/unit_tests/transformer/test_multi_token_prediction.py::TestMultiTokenPrediction::test_forward_backward[1-4-True]
FAILED tests/unit_tests/transformer/test_multi_token_prediction.py::TestMultiTokenPrediction::test_forward_backward[2-1-True]
FAILED tests/unit_tests/transformer/test_multi_token_prediction.py::TestMultiTokenPrediction::test_forward_backward[2-2-True]
FAILED tests/unit_tests/transformer/test_multi_token_prediction.py::TestMultiTokenPrediction::test_forward_backward[2-4-True]
FAILED tests/unit_tests/transformer/test_multi_token_prediction.py::TestMultiTokenPrediction::test_forward_backward[4-1-True]
FAILED tests/unit_tests/transformer/test_multi_token_prediction.py::TestMultiTokenPrediction::test_forward_backward[4-2-True]
FAILED tests/unit_tests/transformer/test_multi_token_prediction.py::TestMultiTokenPrediction::test_fp8_support[True]
FAILED tests/unit_tests/transformer/test_multi_token_prediction.py::TestMultiTokenPrediction::test_packed_sequences_with_full_recompute
```

**Steps/Code to reproduce bug**

Re-run the failing CI job linked above, or locally inside the dev container:

```bash
pytest tests/unit_tests/transformer/test_multi_token_prediction.py::TestMultiTokenPrediction::test_forward_backward
```

**Additional context**

Triaged automatically via `/create-issue`. Assigned to @BestJuly as the author of the refactor that dropped the `padding_mask` parameter; the fix is either to add `padding_mask` back to `_checkpointed_forward` (forwarding it into `_proj_and_transformer_layer`), or to drop the `padding_mask=padding_mask` argument at the call site if the recompute path doesn't need it.