- Drop support for Torch 1.10-1.12, which complexify rules for functorch.
- Make "torch", "torch1" and "torch2" specifiers deal merely with Torch,
  leaving Opacus to the "dp" specifier, as should be.
- Have functorch be installed together with torch, removing some burden
  from our specifier, and enabling support for the new Torch 2.1.

Add 'verbose' argument to 'FederatedClient' and 'TrainingManager'

See merge request !59

Enable recording training loss values

See merge request !58

- Until now, the public methods for training and evaluation using
  the `declearn.main.utils.TrainingManager` class (and its DP-SGD
  counterpart) required message-wrapped inputs and emitted similar
  outputs.
- With this commit, the previously-private `train_under_constraints`
  and `evaluate_under_constraints` routines are made public and thus
  part of the declearn API, with some minor refactoring to make them
  more user-friendly.
- The rationale of this change is to enable using `TrainingManager`
  outside of our `FederatedClient`/`FederatedServer` orchestration,
  notably when simulating FL training or testing client-side code.
  It may also be helpful to end-users that would like to build on
  declearn but implement their own orchestration tools or algorithm
  loops.
- In the future (declearn >=3.0), we may go one step further and
  take all the messaging-related instructions out of the current
  class. The class may also be moved to a different namespace, e.g.
  a new 'declearn.train' module; but this is entirely out of scope
  for now.

- The use of 'functorch.compile' over a function that takes variable-size
  batch inputs proves impossible, as the tracing on first call creates a
  computation graph with fixed dimensions.
- As a result, the tentative compilation of per-sample clipped gradients
  computation prevents the proper use of DP-SGD with the functorch backend.
- An alternative attempt was to compile the sample-wise function and vmap
  it afterwards, but this is currently unsupported (and unlikely to be as
  part of functorch, as development efforts have moved to 'torch.func').
- This commit therefore drops the use of 'functorch.compile'.

Improve tests coverage and fix test-digged bugs

See merge request !57