Weighted Cross Entropy Loss

### 🚀 The feature, motivation and pitch

Allow for passing a weighting tensor to weight the CEL similar to C-RLFT where some tokens or inputs in the batch may have lower weighting to the overall loss. see https://arxiv.org/abs/2309.11235

### Alternatives

_No response_

### Additional context

_No response_

No files, tests, or entry points are identified in the issue. Start by locating the cross-entropy loss implementation and its tests, then review the linked C-RLFT paper; done means the loss accepts a weighting tensor for tokens or inputs and the behavior is covered by tests.

Written by the indexing model from the issue text.