Seed NumPy using np.random.SeedSequence() in pl_worker_init_function() to robustly seed NumPy-dependent dataloader workers

[amorehead]

What does this PR do?

Fixes an issue where small changes to a user's random seed (specified via seed_everything(), e.g., incrementing seed by 1) would not result in a new (unique) random seed for NumPy (set within pl_worker_init_function()). This could cause multi-process dataloader workers to not change their random state based on a user's desired seed set via seed_everything() before training. In other words, if users were relying on seed_everything() to change e.g., the order of the indices sampled by each dataloader worker (if their dataloader(s) use NumPy's random module for index sampling), this would not work until now.

Below is an example of a seed_sequence that previously would not have generated a distinct random seed for NumPy. For example, 7768447584330995212 & 0xFFFFFFFF and 13249712275147347468 & 0xFFFFFFFF yield the same seed even though these former numbers were generated using different base_seeds. To reproduce more of these issues, set base_seed=42,worker_id=0,global_rank=0 within seed_sequence = _generate_seed_sequence(base_seed, worker_id, global_rank, count=4) and then trying incrementing or decrementing base_seed. Most of these small changes to base_seed yield the same random seed for NumPy.

0 =
7900221878587883521
1 =
15926613439302434606
2 =
2812539921844481303
3 =
7768447584330995212

and

0 =
14998723003968126977
1 =
17951608614340099886
2 =
3289043221663264023
3 =
13249712275147347468

-> 

seed_sequence[3] & 0xFFFFFFFF
1078613516

???

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📚 Documentation preview 📚: https://pytorch-lightning--20369.org.readthedocs.build/en/20369/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88%. Comparing base (28d3afd) to head (b9ba94c).
Report is 7 commits behind head on master.

Additional details and impacted files

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[lantiga]

Nice catch! Can you please add a test that verifies the desired behavior so we know it won't regress in the future?