Add serialization function for StaticCache

[xadupre]

What does this PR do?

Implements serialization functions for StaticCache similar to the one implemented for DynamicCache. Fixes pytorch/pytorch#155862.

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[Rocketknight1]

cc @gante!

[justinchuby]

I think #38917 would be possible with this

[gante]

In general LGTM, as it enables a relevant feature.

However, I have two minor grudges :D It would be great if we could solve them, for long-term scalability of the library:

1. This is torch.export-related code that runs on basic imports, regardless of torch.export being used or not. For instance, from transformers import AutoModelForCausalLM will run these lines, which seems wasteful. Is there some way to enable lazy execution? Ideally, these would only be run at export time.
2. This torch.export-related code lives alongside the cache definition, which hurts readability for all other users. Ideally, this code would live in transformers/integrations/export.py (or torch_export.py), but we would have to careful with circular imports.

[gante]

We're moving all these flags to is_torch_greater_or_equal, which is already imported here
e.g. is_torch_greater_or_equal("2.7.0"), or is_torch_greater_or_equal("2.7.0", accept_dev=True) if you also want to accept dev versions of 2.7

[xadupre]

I saw this snippet of code. Do you want me to remove the flag I inserted and use is_torch_greater_or_equal("2.7", accept_dev=True) directly in the code?

is_torch_greater_or_equal_than_2_7 = is_torch_greater_or_equal("2.7", accept_dev=True)  # the line I added
is_torch_greater_or_equal_than_2_6 = is_torch_greater_or_equal("2.6", accept_dev=True)

[justinchuby]

@tugsbayasgalan

[xadupre]

Replaced by #39931.