Update attention_backends.md to update FA3 minimum support to Ampere (#13283)
* Update attention_backends.md
* Update docs/source/en/optimization/attention_backends.md
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>
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Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>