Addition of NLLoss and its variant

[Saanidhyavats]

@vroulet I was thinking that addition of NLLoss (https://pytorch.org/docs/stable/generated/torch.nn.NLLLoss.html), PoissonNLLLoss(https://pytorch.org/docs/stable/generated/torch.nn.PoissonNLLLoss.html) and GaussianNLLLoss(https://pytorch.org/docs/stable/generated/torch.nn.GaussianNLLLoss.html) in this library will be useful.

[vroulet]

Hello @Saanidhyavats,
It makes sense indeed. Would you like to contribute to such a PR?
Make sure to take a look at the other losses to see some template and also to see if these losses could be instantiated through other losses (I don't think so from a quick look but better to verify thoroughly).

[Saanidhyavats]

Sure I will create a PR for this