No such operator custom::warp_perspective error with building-with-setuptools example

[deepali-c]

While executing the https://pytorch.org/tutorials/advanced/torch_script_custom_ops.html#building-with-setuptools example had observed the following error:

>>> import torch
>>> torch.ops.load_library("warp_perspective.so")
>>> print(torch.ops.custom.warp_perspective)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<myenv>/lib/python3.6/site-packages/torch/_ops.py", line 61, in __getattr__
    op = torch._C._jit_get_operation(qualified_op_name)

RuntimeError: No such operator custom::warp_perspective