Deterministic dropout layer

[mryab]

Implements custom dropout layer accepting precomputed dropout mask as an input, tests its determinism (although it can't fail in any conceivable situation). Also fixed some bugs/inconsistencies in backend/utils/test_utils which I faced when writing a test.

Hopefully solves #21.

TODO:

• Custom DeterministicDropout function and layer
• Tests for forward/backward determinism
• Add docstrings for DeterministicDropout
• Rewrite all docstrings hinting at non-determinism (e.g. here and here). From now, users will have to implement custom layers with no state other than parameters and explicitly track all buffers/RNG outputs if they want complete determinism.