Replace flash-attn with kernels in CI

[albertvillanova]

Replace flash-attn with kernels in CI.

As suggested by @qgallouedec: #4297 (comment)

A lot of our tests rely on flash-attn lib; it's probably a good time to drop flash-attn and rely only on kernels:

• replace all attn_implementation="flash_attention_2" -> attn_implementation="kernels-community/flash-attn"
• replace require_flash_attn by require_kernels