Optimize ForCausalLMLoss by removing unnecessary contiguous() call to reduce memory overhead
Author: efsotr
Pull request: yes

# What does this PR do?

Removing unnecessary contiguous() call

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Comments:
**Condition:** Without clearing the cache, the `dtype` is `bfloat16`, and the size of `logits` is 0.5 GB.  
The old `ForCausalLMLoss` requires 3 GB for the forward pass and an additional 0.5 GB to hold the original `logits`.  
The new `ForCausalLMLoss` reduces this to 2 GB for the forward pass while still needing 0.5 GB for holding the `logits`.  
For the backward pass, both versions require 3 GB, plus the 0.5 GB needed to hold the original `logits`.


Thank you very much, I want to understand some questions.Can you explain some of your test case,I don't understand what's the meaning of "~create_mask(max_seqlen, half_prompts_lens)" and "mask" and "start_pos_id" and "end_pos_id". Can you give me your personal email? @efsotr 

(Merging without @ArthurZucker approval because the code has the same output, it's just faster + less memory)

LGTM anyways, thanks all 🤗