Add loguru to requirements.txt

[ez2rok]

When I run

python scripts/train.py --config configs/train_meru_vit_l.py --num-gpus 8 --output-dir ./output

I get the error

Traceback (most recent call last):
  File "/home/ethan/meru/scripts/train.py", line 17, in <module>
    from loguru import logger
ModuleNotFoundError: No module named 'loguru'

It seems like we should add loguru to the requirements.txt file.

Cheers!