feat: add lazy imports for faster startup time

[Erotemic]

For easier review, this is broken out feature I wrote in #150

This uses lazy imports

Issue Being Solved

The problem is that the startup time of the yolo/lazy.py script is very long because yolo/__init__.py explicitly imports a lot of heavyweight dependencies.

Previously running: time python -m yolo.lazy --help took 3 seconds on my machine. After this patch it now takes 0.3 seconds, a 10x improvement.

It works using the lazy-loader module. Instead of explicitly importing everything, it only will import them if they are used. You can also specify the EAGER_IMPORT=1 environment variable to have it import everything on startup as it used to. E.g, the following command will revert to the old 3 second startup time:

time EAGER_IMPORT=True python -m yolo.lazy --help

Additionally I had to move a few imports in lazy.py itself into the main function so it can hit the hydra code before it starts to import all of lightning.

Note on mkinit

Note, that I wrote this using mkinit autogeneration, which is why there is a __submodules__ and __autogen__ definition at the top. Technically this can be removed. But it also lets you regenerate the __init__.py file more easily if anything changes. It can even reconstruct the original explicit definition of the file with:

mkinit ./yolo/__init__.py --nomods --write

On Dependencies

This does add a dependency on lazy-loader, but that could be removed by defining the lazy_loader.attach directly in the __init__.py, but it is a bit ugly, so I tend to prefer using the lightweight package. Running mkinit with --lazy instead of --lazy-loader will generate this variant of the code.

[henrytsui000]

I'm sorry for not being familiar with lazy_loader. Give me some time to review the code! The import process takes quite a long time and is indeed a bit annoying for me as well XD. I think this will be really helpful!

[madmike2010]

https://peps.python.org/pep-0008/#imports

Imports are always put at the top of the file, just after any module comments and docstrings, and before module globals and constants.

[Erotemic]

In this case it is done due a quirk of how mkinit autogenerates code, so if you want the lazy-loader dependency path I can fix that. Otherwise, that would be replaced with a definition of the function that does the same thing. Technically none of the constants are needed, they are just convenient to automatically update the __init__.py with mkinit.

However, it's important to note: always adhering to this rule can have significant startup-time consequences. I moved some imports after the call to setup inside of the main function here: https://github.com/MultimediaTechLab/YOLO/pull/160/files#diff-029d4708225fe34b2045fd6a94297034336187c511ca506dccd687f5b6fca3c3R16 and this is important for a faster startup time.