Allow the option for timestamp to be generated for ns-train --load-config

[kobejean]

When running ns-train --load-config $CONFIG_PATH currently the timestamp is fixed to what was in the config file, which is probably fine. But when I omit the timestamp parameter from the config file, it would be nice to have a new timestamp be set. Instead, the current implementation leaves the timestamp value as the default string value "{timestamp}" instead of a new valid timestamp.

I believe we can fix this with a single line change, by moving the call to config.set_timestamp() to after loading the config file. I don't see how this will impact anything else.

This won't affect people who want their timestamp fixed to what the config file reads because config.set_timestamp() only sets the value if it is set to the default value "{timestamp}" due to the if statement here:

nerfstudio/nerfstudio/configs/experiment_config.py

Lines 97 to 100 in ef2fd3d

	def set_timestamp(self) -> None:
	"""Dynamically set the experiment timestamp"""
	if self.timestamp == "{timestamp}":
	self.timestamp = datetime.now().strftime("%Y-%m-%d_%H%M%S")

I think this behavior is probably was what was intended originally but got lost overtime.

I have tested this locally on an ubuntu computer.

Please consider this PR

Cheers