[webgpu] make PIX frame generator per session (#26873)
### Description
As a following-up of #26855, this PR changes the PIX fram generator from
per-WebGpuContext to per inference session, to support more flexible
control for performance diagnositcs.
### 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. -->
