add causal_upper_left mask option to scaled_dot_product_attention

[mm65x]

Proposed changes

#2835

adds "causal_upper_left" and "causal_lower_right" as explicit mask options
to mx.fast.scaled_dot_product_attention. "causal" stays as an alias for
"causal_lower_right", so nothing breaks.

when S_Q != S_KV, lower-right aligns the last query with the last key,
while upper-left aligns query i with keys 0..i (matching PyTorch's
is_causal=True). when S_Q == S_KV they're identical.

on Metal, the full-attention kernels already parameterize the causal diagonal
via qL_off in AttnParams, so the change is just passing 0 instead of
kL - qL for upper-left. the vector kernels previously hardcoded the offset
inline, so a causal_offset buffer argument was added instead. on CUDA, the
cuDNN path uses set_causal_mask vs set_causal_mask_bottom_right, and the
vector kernels use a causal_offset field in AttnParams (same approach as
Metal).

Checklist

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)

[zcbenz]

Can you fix the lint error?