Add PT compileable support for flash_attn_with_kvcache

[jataylo]

Continues #1139 adding custom op for flash_attn_with_kvcache.

On a transformers model this improves perf by >2x by avoiding graph breaks. There is a gotcha here, with this implementation an error is thrown in PyTorch 2.6 in user code when reshaping FA output:

torch._dynamo.exc.BackendCompilerFailed: backend='inductor' raised:
RuntimeError: <weakref at 0x7f10e00494e0; to 'torch.storage.UntypedStorage' at 0x7f10e0049400>

This is not an issue for PyTorch 2.7, so I had to introduce conditionalisation to workaround this by returning clone of the output tensors only for PT versions earlier than 2.7 and when compile is being used.

[jataylo]

@tridao alternatively if preferred, instead of conditionalising the clone for < PT 2.7, we could just disable compile-able support for this op if below 2.7, the additional clone could cause regressions and increase memory usage.

[tridao]

We will drop support for pytorch < 2.4 so you can simplify the code.
I'll need to think more about the clone. Does it slow things down when running in eager?