Added None to act as max for np.clip

[GeorgePearse]

Added None to act as max for np.clip

Summary:

I was receiving an error due to the lack of a max defined for np.clip().

Traceback (most recent call last):
  File "main.py", line 226, in <module>
    run(model, baal_trainer, baal_data_module, config, experiment_name)
  File "main.py", line 212, in run
    should_continue = baal_trainer.step(model, datamodule=baal_data_module)
  File "/workspace/environments/active-learning/lib/python3.8/site-packages/baal/utils/pytorch_lightning.py", line 202, in step
    to_label = self.heuristic(probs)
  File "/workspace/environments/active-learning/lib/python3.8/site-packages/baal/active/heuristics/heuristics.py", line 258, in __call__
    return self.get_ranks(predictions)[0]
  File "/workspace/environments/active-learning/lib/python3.8/site-packages/baal/active/heuristics/stochastics.py", line 45, in get_ranks
    distributions = np.clip(distributions, 0)
  File "<__array_function__ internals>", line 4, in clip
TypeError: _clip_dispatcher() missing 1 required positional argument: 'a_max'

Features:

Max defined as None for np.clip

Checklist:

• Your code is documented (To validate this, add your module to tests/documentation_test.py).
• Your code is tested with unit tests.
• You moved your Issue to the PR state.

NB: sorry this is a comically small PR but just wanted to go through the motions with one. Will try to address the string mapping for PowerSampling etc. on Sunday but no promises.

I haven't yet run the tests, but have used it.

[Dref360]

LGTM thanks!