fix sdpa attn mask with 1 channel

[nihui]

No description provided.

[tencent-adm]

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.59%. Comparing base (d73c0d6) to head (bdbf4ba).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6402      +/-   ##
==========================================
- Coverage   95.88%   95.59%   -0.30%     
==========================================
  Files         842      842              
  Lines      266551   266551              
==========================================
- Hits       255595   254809     -786     
- Misses      10956    11742     +786     

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[github-actions]

The binary size change of libncnn.so (bytes)

architecture	base size	pr size	difference
x86_64	15234680	15234680	0 😘
armhf	6220404	6220452	+48 ⚠️
aarch64	9525552	9525456	-96 😘