Suggestion of multiprocess mechanism in MultiThreadedAugmenter

[SeanCho1996]

Hi, I noticed that the finish procedure in MultiThreadedAumenter uses the terminate() method of Process to end the child process by sending SIGTERM.

batchgenerators/batchgenerators/dataloading/multi_threaded_augmenter.py

Lines 273 to 275 in 01f225d

	if len(self._processes) != 0:
	logging.debug("MultiThreadedGenerator: shutting down workers...")
	[i.terminate() for i in self._processes]

In my project, my main process has a sigterm-handler set up, which was meant to stop the process via SIGTERM at the end of my training, shown as follow:

 def _sigterm_handler(_signo, _stack_frame):
        logger.warn("Terminal signal received: %s, %s" % (_signo, _stack_frame))
        stop_worker()
        exit(0)

However, the following problem occurs when working with MultiThreadedAugmenter's terminate(): when the child process is created, it forks all the methods of the main process, including my sigterm-handler, which causes MultiThreadedAugmenter's ending SIGTERM will be caught by the sigterm-handler, which will directly end my training process.

A temporary solution I came up with is to override the child process with a default signal handler signal.SIG_DFL, so that the SIGTERM of the child process does not trigger the sigterm-handler forked from main process, which means adding one line at the beginning of producer() function:

def producer(queue, data_loader, transform, thread_id, seed, abort_event, wait_time: float = 0.02):
    signal.signal(signal.SIGTERM, signal.SIG_DFL)
    ...

Is it possible that a similar operation needs to be added to the source code to avoid the impact of the child process signal on the main process？

Thank you