Fix: explicit not none check for tensors in flash attention
Author: jeffrey-dot-li
Pull request: yes

# What does this PR do?

Current use_mask check in flash attention:
`use_mask = position_ids is not None or all([cu_seq_lens_q, cu_seq_lens_k, max_length_q, max_length_k])`
throws error
```
    use_mask = position_ids is not None or all([cu_seq_lens_q, cu_seq_lens_k, max_length_q, max_length_k])
                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
RuntimeError: Boolean value of Tensor with more than one value is ambiguous
```

Replace with explicit not None check:
`    use_mask = position_ids is not None or all(k is not None for k in [cu_seq_lens_q, cu_seq_lens_k, max_length_q, max_length_k])`

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Fixes # (issue)


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## Who can review?
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