Fix _generate_seed_sequence sampling

[SkafteNicki]

What does this PR do?

Fixes #20412
Currently the _generate_seed_sequence function does not produce unique seeds. The reason for this is that small base seeds (e.g. 0-999) after left-shifting by 32 bits have all zeros in the lower 32 bits. The LCG algorithm currently being used with c=1, then produces seeds that all have lower_32_bits=1 which in turn causes torch.manual_seed() to treat them identically since Pytorch primarily uses the lower bits.

Implements a simple hash mixing that ensures even small seeds produce well-distributed bit patterns across the full 64-bit space, preventing the current seed collision we are seeing.

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📚 Documentation preview 📚: https://pytorch-lightning--21399.org.readthedocs.build/en/21399/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87%. Comparing base (79ffe50) to head (d451eca).

Additional details and impacted files

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- Coverage      87%      87%   -0%     
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  Files         269      269           
  Lines       23804    23809    +5     
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+ Hits        20626    20628    +2     
- Misses       3178     3181    +3