add torch.positive

[nicochen1118]

No description provided.

[tencent-adm]

All committers have signed the CLA.

[nicochen1118]

Sorry, why the checks were not successful?

[nihui]

Sorry, why the checks were not successful?

torch-1.8 does not have torch.positive
you can move to test to conditional section, like F.mish

 Traceback (most recent call last):
  File "/data/devops/workspace/tools/pnnx/tests/test_torch_positive.py", line 58, in <module>
    if test():
  File "/data/devops/workspace/tools/pnnx/tests/test_torch_positive.py", line 38, in test
    a = net(x, y, z)
  File "/data/devops/workspace/torch-1.8.1/lib/python3.9/site-packages/torch/nn/modules/module.py", line 889, in _call_impl
    result = self.forward(*input, **kwargs)
  File "/data/devops/workspace/tools/pnnx/tests/test_torch_positive.py", line 24, in forward
    x = torch.positive(x)
AttributeError: module 'torch' has no attribute 'positive'
CMake Error at /data/devops/workspace/tools/pnnx/tests/run_test.cmake:5 (message):
  Test failed with return value '1'

[nihui]

Thanks for your contribution !