[webgpu] support auto pad for im2col-matmul (#26771)
### Description
<!-- Describe your changes. -->
This PR addressed the TODO about auto_pad for `im2col-matmul`, because
the auto_pad attribute was used for computing the padding for the Conv2d
before launching the kernel, so we don't need to do extra work to
support it in the logic of the new `im2col-matmul` kernel.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Please see above.
