[Dev] Remove calculation of padding token in moe routing loss

[HaochenYuan]

What does this PR do ?

Fix error in mlp recompute with padding mask in previous MR 2121
Related issue: #1984
PR to the main branch: #2142

When enabling mlp recompute, got below functional test error:

File "/opt/megatron-lm/megatron/core/transformer/transformer_layer.py", line 674, in _forward_mlp
[default0]:[rank0]:     mlp_output_with_bias = tensor_parallel.checkpoint(
[default0]:[rank0]:                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^
[default0]:[rank0]: TypeError: checkpoint() got an unexpected keyword argument 'padding_mask'

Fixed it by binding kwargs to recompute function before calling CheckpointFunction.apply

mlp_output_with_bias = tensor_parallel.checkpoint(
                    functools.partial(self.mlp, padding_mask=padding_mask),
                    False,
                    pre_mlp_layernorm_output,
                )

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flowchart LR
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