Update basnet_test.py

[bikcrum]

Description

When python basnet_test.py was executed, the subprocesses were created recursively and execution was failed with an error. The solution to this problem is to wrap the code in if __name__ == '__main__':

Stacktrace:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/proshore/opt/anaconda3/lib/python3.8/multiprocessing/spawn.py", line 116, in spawn_main
    exitcode = _main(fd, parent_sentinel)
  File "/Users/proshore/opt/anaconda3/lib/python3.8/multiprocessing/spawn.py", line 125, in _main
    prepare(preparation_data)
  File "/Users/proshore/opt/anaconda3/lib/python3.8/multiprocessing/spawn.py", line 236, in prepare
    _fixup_main_from_path(data['init_main_from_path'])
  File "/Users/proshore/opt/anaconda3/lib/python3.8/multiprocessing/spawn.py", line 287, in _fixup_main_from_path
    main_content = runpy.run_path(main_path,
  File "/Users/proshore/opt/anaconda3/lib/python3.8/runpy.py", line 265, in run_path
    return _run_module_code(code, init_globals, run_name,
  File "/Users/proshore/opt/anaconda3/lib/python3.8/runpy.py", line 97, in _run_module_code
    _run_code(code, mod_globals, init_globals,
  File "/Users/proshore/opt/anaconda3/lib/python3.8/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/Users/proshore/Documents/work/python/ai/BASNet/basnet_test.py", line 75, in <module>
    for i_test, data_test in enumerate(test_salobj_dataloader):
  File "/Users/proshore/opt/anaconda3/lib/python3.8/site-packages/torch/utils/data/dataloader.py", line 352, in __iter__
    return self._get_iterator()
  File "/Users/proshore/opt/anaconda3/lib/python3.8/site-packages/torch/utils/data/dataloader.py", line 294, in _get_iterator
    return _MultiProcessingDataLoaderIter(self)
  File "/Users/proshore/opt/anaconda3/lib/python3.8/site-packages/torch/utils/data/dataloader.py", line 801, in __init__
    w.start()
  File "/Users/proshore/opt/anaconda3/lib/python3.8/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
  File "/Users/proshore/opt/anaconda3/lib/python3.8/multiprocessing/context.py", line 224, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
  File "/Users/proshore/opt/anaconda3/lib/python3.8/multiprocessing/context.py", line 283, in _Popen
    return Popen(process_obj)
  File "/Users/proshore/opt/anaconda3/lib/python3.8/multiprocessing/popen_spawn_posix.py", line 32, in __init__
    super().__init__(process_obj)
  File "/Users/proshore/opt/anaconda3/lib/python3.8/multiprocessing/popen_fork.py", line 19, in __init__
    self._launch(process_obj)
  File "/Users/proshore/opt/anaconda3/lib/python3.8/multiprocessing/popen_spawn_posix.py", line 42, in _launch
    prep_data = spawn.get_preparation_data(process_obj._name)
  File "/Users/proshore/opt/anaconda3/lib/python3.8/multiprocessing/spawn.py", line 154, in get_preparation_data
    _check_not_importing_main()
  File "/Users/proshore/opt/anaconda3/lib/python3.8/multiprocessing/spawn.py", line 134, in _check_not_importing_main
    raise RuntimeError('''
RuntimeError: 
        An attempt has been made to start a new process before the
        current process has finished its bootstrapping phase.

        This probably means that you are not using fork to start your
        child processes and you have forgotten to use the proper idiom
        in the main module:

            if __name__ == '__main__':
                freeze_support()
                ...

        The "freeze_support()" line can be omitted if the program
        is not going to be frozen to produce an executable.
Traceback (most recent call last):
  File "/Users/proshore/opt/anaconda3/lib/python3.8/site-packages/torch/utils/data/dataloader.py", line 872, in _try_get_data
    data = self._data_queue.get(timeout=timeout)
  File "/Users/proshore/opt/anaconda3/lib/python3.8/multiprocessing/queues.py", line 107, in get
    if not self._poll(timeout):
  File "/Users/proshore/opt/anaconda3/lib/python3.8/multiprocessing/connection.py", line 257, in poll
    return self._poll(timeout)
  File "/Users/proshore/opt/anaconda3/lib/python3.8/multiprocessing/connection.py", line 424, in _poll
    r = wait([self], timeout)
  File "/Users/proshore/opt/anaconda3/lib/python3.8/multiprocessing/connection.py", line 931, in wait
    ready = selector.select(timeout)
  File "/Users/proshore/opt/anaconda3/lib/python3.8/selectors.py", line 415, in select
    fd_event_list = self._selector.poll(timeout)
  File "/Users/proshore/opt/anaconda3/lib/python3.8/site-packages/torch/utils/data/_utils/signal_handling.py", line 66, in handler
    _error_if_any_worker_fails()
RuntimeError: DataLoader worker (pid 73291) exited unexpectedly with exit code 1. Details are lost due to multiprocessing. Rerunning with num_workers=0 may give better error trace.

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "basnet_test.py", line 75, in <module>
    for i_test, data_test in enumerate(test_salobj_dataloader):
  File "/Users/proshore/opt/anaconda3/lib/python3.8/site-packages/torch/utils/data/dataloader.py", line 435, in __next__
    data = self._next_data()
  File "/Users/proshore/opt/anaconda3/lib/python3.8/site-packages/torch/utils/data/dataloader.py", line 1068, in _next_data
    idx, data = self._get_data()
  File "/Users/proshore/opt/anaconda3/lib/python3.8/site-packages/torch/utils/data/dataloader.py", line 1034, in _get_data
    success, data = self._try_get_data()
  File "/Users/proshore/opt/anaconda3/lib/python3.8/site-packages/torch/utils/data/dataloader.py", line 885, in _try_get_data
    raise RuntimeError('DataLoader worker (pid(s) {}) exited unexpectedly'.format(pids_str)) from e
RuntimeError: DataLoader worker (pid(s) 73291) exited unexpectedly

[bikcrum]

@xuebinqin May you please review this?

[xuebinqin]

merged, thanks for your pr.