[Vulkan] Move convert_qconv2d_context to custom ops (#98548)
Summary: Move convert_qconv2d_context to it's own custom op library
Test Plan: ```buck run --target-platforms ovr_config//platform/macos:arm64-fbsource -c pt.vulkan_full_precision=1 //xplat/caffe2/fb/custom_ops/vulkan_quantized:pt_vulkan_quantized_test_binAppleMac\#macosx-arm64```
Differential Revision: D44688797
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98548
Approved by: https://github.com/kirklandsign
