[webgpu] Register GQA based on graph capture (#26384)
This pull request enables conditionally register GQA with
total_sequence_length on gpu or not. It resolves the issue that a
MemcpyToHost is generated when graph capture is enabled (refer to
#25868). This is the last functionality part to support graph capture in
webgpu ep in ORT.
The main changes ensure that when graph capture is enabled, sequence
length information is read from GPU buffers instead of CPU memory, and
shader code generation adapts accordingly. This enables more efficient
execution and compatibility with graph-captured models.
In this PR, we still get total sequence length from `seqlen_k` tensor
not `total_seqlen_tensor` tensor to keep consistent with other parts. In
the next PR, we can refactor all places to directly use
`total_seqlen_tensor` instead of `seqlen_k` when graph capture enabled.
