[fix] avoid too small block m/n for flex attention

[AlpinDale]

No description provided.

[gemini-code-assist]

Code Review

This pull request introduces a fix to prevent FlexAttention block sizes (block_m and block_n) from becoming too small by enforcing a lower bound of 16. This is a good fix to improve the stability of the attention kernel. My review focuses on improving the code's maintainability by replacing the magic number 16 with a named constant, which makes the code clearer and easier to manage in the future.

[gemini-code-assist]

The value 16 is a magic number. To improve readability and maintainability, it should be defined as a constant with a descriptive name in UPPER_CASE_WITH_UNDERSCORES as per PEP 8. This makes the code's intent clearer and future updates easier. The suggestion below introduces a constant and assigns it to block_lower_bound for compatibility with the rest of the code in this PR. Ideally, block_lower_bound should be replaced with the constant MIN_KERNEL_BLOCK_SIZE directly on lines 850-851.

Suggested change

	block_lower_bound = 16
	MIN_KERNEL_BLOCK_SIZE = 16
	block_lower_bound = MIN_KERNEL_BLOCK_SIZE