[nll_loss] Avoid unnecessary type casts

[crcrpar]

follow-up #85395

AT_DISPATCH_NLL_LOSS_INDEX_TYPES should not be removed in favor of #59765 and there's a testcase

pytorch/test/test_nn.py

Line 16832 in 99ca25e

def test_nll_loss_byte_target_matches_long(self, device):

Besides the dispatcher, I wanted to sanity check int64_t ignore_index because int64_t can be inappropriate considering that target can be Byte. However, given that the default value is -100 as in

pytorch/aten/src/ATen/native/native_functions.yaml

Line 9949 in 0a75c42

- func: nll_loss(Tensor self, Tensor target, Tensor? weight=None, int reduction=Mean, int ignore_index=-100) -> Tensor

it's not easy to add a check while keeping the backward compatibility. Thus I decided to not add a check.

cc @lezcano @t-vi

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[lezcano]

Cool, thank you!

[crcrpar]

@pytorchbot merge -g

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Hey @crcrpar.
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