add F_pairwise_distance to pnnx and ncnn

[Marsyule]

had added to pnnx

[tencent-adm]

All committers have signed the CLA.

[nihui]

hi, please enable github action in your forked repo, to let code-format job work properly

[nihui]

please fix ci error

  36/506 Test  #50: test_F_pairwise_distance ......................***Failed    0.06 sec
  File "/data/devops/workspace/tools/pnnx/tests/test_F_pairwise_distance.py", line 52
    return torch.equal(a0,b0) and torch.equal(a1,b1) and torch.equal(a2,b2) and torch.equal(a3,b3)
IndentationError: unexpected indent
CMake Error at /data/devops/workspace/tools/pnnx/tests/run_test.cmake:5 (message):
  Test failed with return value '1'

[nihui]

overall it looks good besides some space style issues

[nihui]

There isn't a ncnn layer called PairwiseDistance.
Either implement the layer or drop the ncnn conversion and leave pnnx conversion only.

If you plan to implement the ncnn layer later, add the ncnn conversion at that time.

[nihui]

remove this source as no such ncnn layer exists

[nihui]

Thanks for your contribution !