Add manager._start_time to state_dict

[linshokaku]

pytorch-pfn-extras/pytorch_pfn_extras/training/manager.py

Lines 642 to 644 in 2dcd7d7

	if self._start_time is None:
	self._start_time = _get_time()
	self.start_extensions()

pytorch-pfn-extras/pytorch_pfn_extras/training/manager.py

Lines 233 to 239 in 2dcd7d7

	@property
	def elapsed_time(self) -> float:
	if self._start_time is None:
	raise RuntimeError(
	"Unavailable until the initial run_iteration call."
	)
	return _get_time() - self._start_time

When restarting from a snapshot, _start_time is initialized and elapsedtime cannot be obtained correctly, so _start_time should be treated as state.

[kmaehashi]

If we are to fix this, I think it's better to record elapsed_time in state dict and accumulate the "real" time consumed (i.e., excluding the time process is stopped)