Improvement: Update normalization in ACT + remove in-place modification + add test

What this does

This PR work on the Normalization in ACT to:

remove the default added epsilon in favor of capping the std (and min difference between max/min) to a epsilon value
make the normalization not in-place
add test for the bahavior of the normalization

The reasons for these proposals are as follow:

remove the default added epsilon in favor of capping the std:
- in both torchvision and the original ACT code a capping of the min value of std is used instead of a constantly added epsilon. this bring us in-line with what other well-used repos are doing
- in most cases std is not null we are thus closer to the theoretical behavior for the normalization
make the normalization not in-place
- right now the normalization was returning a tensor but also modifying it in-place instead the function. This can be error-prone as the user typically doesn't expect a function returning a value to also modify the input in-place.
- for now I decided to make it not in-place. An alternative could be to make it fully in-place and remove the returned value. Less intuitive according to me for probably minor speed/memory gains

Added extensive tests for the behavior in tests/test_policies.py::test_normalize

Run the tests:

pytest ./tests/test_policies.py::test_normalize

This change is