Generation of lmdb database does not work for the current version of lmdb

[ChristianBerwanger]

Hello,
When creating my own ImageNet LMDB with the provided imagenet_lmdb_dataset (in mambavision/utils/datasets.py) code, I ran into two issues:

1. Type Error: lmdb expects an int as the map_size, but 1e12 is interpreted as a float.
2. Environment Error: The script opens the dataset a second time before closing it, which results in an lmdb.Error: The environment '' is already open in this process error.

I have a local fix for both. Would you be open to a Pull Request (PR)?

Kind regards,
Christian