fix flex attn when optional args aren't passed
Author: winglian
Pull request: yes
Labels: name: for patch

# What does this PR do?
#37307 (llama4) introduced a regression in make_flex_block_causal_mask with new optional args that llama4 uses. It also added padding, but padding only works if the `key_length` isn't None. For all models except llama4, flex attention is now broken since none of them pass key_length, and currently they error with.

```
[rank3]:   File "/root/miniconda3/envs/py3.11/lib/python3.11/site-packages/torch/nn/functional.py", line 5209, in pad                                                                                                                                        
[rank3]:     return torch._C._nn.pad(input, pad, mode, value)                                                                                                                                                                                                
[rank3]:            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                                                                                                                                                                                
[rank3]: TypeError: pad(): argument 'pad' failed to unpack the object at pos 2 with error "type must be tuple of ints,but got NoneType"                                                                                                                      
```

Also, the batch size was likely mistakenly hard-coded to 1, which probably shouldn't be the case.  

Fixes # (issue)


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Comments:
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Thanks for the prompt fix!