Delay float32 upcast in ForCausalLMLoss after filtering ignore_index
Author: starcatmeow
Pull request: yes

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This PR implements the optimization discussed in #38452, originally proposed by @harshit2997.  
Thanks for the original suggestion and discussion.

- Move the float32 upcast in `ForCausalLMLoss` to after filtering out `ignore_index` labels.
- Ensures only relevant logits are upcasted, reducing VRAM usage without affecting correctness.

Fixes #38452


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Comments:
Hi @starcatmeow, I just took a look and I'm not sure we can accept this! Although I thought it was a good optimization in the issue at #38452, this is because I misread - I thought the code was already selecting masked labels, it was just doing it **after** casting to `float32` instead of before. In that case, doing the select first would save memory with no issues.

However, adding a mask select step when we didn't use one before introduces some problems - in particular, it makes the sizes of the output tensors data-dependent, which can force recompilations with less efficient dynamic shapes. I'm not sure it's worth it for the memory saving here! cc core maintainers @arthurzucker @cyrilvallez for their opinion

Interesting idea, I'm not sure how often we expect to see `ignore_index` in the labels, cc @ArthurZucker do you have an idea? It could actually save a LOT of memory as vocabularies tend to be large now (200k+)

Mathematically, it's fully equivalent

Thanks @starcatmeow for picking this change up. @Cyrilvallez my main motivation behind proposing the change was to account for fine-tuning cases where one doesn't want to fine tune on prompt tokens. In addition, won't this also help with getting rid of padding tokens before the upcast which is a common scenario?