[Feature] Restructure torchrl/objectives

[sgrigory]

Description

Closes #117

Restructures torchrl/objectives folder in line with #152 (comment). Tests, examples, notebooks, and markdown docs which import torchrl.objectives are also changed accordingly.

Motivation and Context

Context: TorchRL comes with a set of stateful loss modules that use helper functions to compute returns and advantages. The current structure does not reflect the dependency of the losses on the helper functions.

This PR moves files:

• from torchrl/objectives/costs to torchrl/objectives
• from torchrl/objectives/returns to torchrl/objectives/value

It also eliminates __all__ = [...] constructions in imported files in torchrl/objectives, instead explicitly listing imported objects in places where imports happen

Types of changes

What types of changes does your code introduce? Remove all that do not apply:

• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)
• Example (update in the folder of examples)

Checklist

• I have read the CONTRIBUTION guide
• My change requires a change to the documentation.
• I have updated the tests accordingly.
• I have updated the documentation accordingly.

Testing plan

• pytest passes locally

• Example scripts in examples/ run

[codecov]

Codecov Report

Merging #580 (4304553) into main (685c08e) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #580      +/-   ##
==========================================
- Coverage   86.96%   86.96%   -0.01%     
==========================================
  Files         121      120       -1     
  Lines       21820    21811       -9     
==========================================
- Hits        18976    18967       -9     
  Misses       2844     2844              

Flag	Coverage Δ
linux-cpu	85.29% <100.00%> (-0.01%)	⬇️
linux-gpu	86.74% <100.00%> (-0.01%)	⬇️
linux-outdeps-gpu	75.39% <100.00%> (-0.02%)	⬇️
linux-stable-cpu	85.27% <100.00%> (-0.01%)	⬇️
linux-stable-gpu	86.74% <100.00%> (-0.01%)	⬇️
macos-cpu	85.05% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
torchrl/objectives/common.py	88.37% <ø> (ø)
torchrl/objectives/utils.py	85.10% <ø> (ø)
torchrl/objectives/value/returns.py	44.44% <ø> (ø)
torchrl/objectives/value/utils.py	84.61% <ø> (ø)
torchrl/objectives/value/vtrace.py	26.08% <ø> (ø)
test/smoke_test.py	100.00% <100.00%> (ø)
test/test_cost.py	96.41% <100.00%> (ø)
torchrl/objectives/__init__.py	100.00% <100.00%> (ø)
torchrl/objectives/ddpg.py	97.87% <100.00%> (ø)
torchrl/objectives/deprecated.py	92.92% <100.00%> (ø)
... and 11 more

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[vmoens]

Great work thanks for this!