[causal_mask] Add a warning when registering the buffer

[ArthurZucker]

What does this PR do?

After #27931, the importance of max_position_embedding has been greatly improved. Before it would cache sin_cache and cos_cache, which are relatively small tensors.
With the update and the need of a static shaped mask, we are no longer caching sin and cos, but are caching the causal_mask.

1. Either we keep this and warn users
2. Or we allocate a new mask at each forward, making sure this is compatible with compile.

Decided to go for 1.

One possible problem is if we re-introduce the sin_cache and cos_cache for performance issues (if it proves to be a lot slower).

[HuggingFaceDocBuilderDev]

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