[webgpu] And int64 to cast

[qjia7]

This pull request extends the WebGPU execution provider to support int64 data type casting in the Cast operator, with conditional support based on whether graph capture is enabled. It refactors kernel registration to allow toggling int64 support and updates the shader code and kernel logic to handle int64 tensors efficiently.

It's part of the work to enable graph capture in phi4 #25868

[qjia7]

Sorry, please hold on the review. I may miss some situations.

[qjia7]

@guschmue @fs-eire This PR is ready for review. Please take a look, thanks.
cc @sushraja-msft @xhcao

[xhcao]

Hi, @qjia7 , is it not very reasonable that bind the usage of Cast(tensor_int64) and enable_graph_capture together, sometimes someone only want (or only can use) Cast(tensor_int64)?
Why not add an extra string list option in SessionOptions, for example add enable_cast_int64 into the string list if want Cast(tensor_int64), and query the string when register EP kernels? It also easily extend to enable_pad_int64, enable_gathernd_int64, etc.
It is my personal point. If other reviewers think the current code is fine, it is also ok for me.

[qjia7]

Hi, @qjia7 , is it not very reasonable that bind the usage of Cast(tensor_int64) and enable_graph_capture together, sometimes someone only want (or only can use) Cast(tensor_int64)? Why not add an extra string list option in SessionOptions, for example add enable_cast_int64 into the string list if want Cast(tensor_int64), and query the string when register EP kernels? It also easily extend to enable_pad_int64, enable_gathernd_int64, etc. It is my personal point. If other reviewers think the current code is fine, it is also ok for me.

Hi Xinghua, this change is target for graph capture and doesn't affect the existing logic. I understand other cases may also need the cast_int64 support in webgpu. But I'm unsure if a session option is the best solution since many other ops may also need such kind of changes. I also offline talked with @fs-eire. He is considering an alternative way to support int64 in GetCapability. Therefore, we need to consider more factors. I prefer to proceed with this PR to unblock progress. Even if the final solution involves adding a session option, it won't affect the main changes of this PR—only the dynamic registration conditions would need to be modified.