Feature/buffer np

[belerico]

Summary

In this PR we have introduced our new buffers, that handle data with standard dictionaries of numpy arrays instead of TensorDicts. This change has been introduced to keep algorithms implementation and interaction as simple as possible, while also gaining in terms of speed performance. In particular:

• All the buffers have been converted to work with standard dictionaries of numpy arrays instead of TensorDicts
• We have introduced the class sheeprl.utils.memmap.MemmapArray, which is a simple container to work with numpy memory-mapped arrays. Those arrays can be easily shared between processes that share the same underlying file-system and once shared they maintain the ownership of the array being shared
• The EpisodeBuffer api has been made compliant to the ReplayBuffer api:
  • the add method takes in input one or more steps (possibly containing multiple episodes) instead of a single episode
• The CheckpointCallback has been adapted to our new buffers

Type of Change

Please select the one relevant option below:

• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

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• I have tested my changes locally and they work as expected. (Please describe the tests you performed.)
• I have added unit tests for my changes, or updated existing tests if necessary.
• I have updated the documentation, if applicable.
• I have installed pre-commit and run locally for my code changes.

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