[Feature] Python-based RNN Modules

[albertbou92]

Description

Implements Python-based RNN Modules

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✅ You can merge normally! (6 Unrelated Failures)

As of commit e1d7311 with merge base 2a72e6d ():

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• Continuous Benchmark (PR) / CPU Pytest benchmark (gh)
• Continuous Benchmark (PR) / GPU Pytest benchmark (gh)
• Habitat Tests on Linux / tests (3.9, 11.6) / linux-job (gh)
• Unit-tests on Windows CPU / unittests / windows-job (gh)
• Unit-tests on Windows GPU / unittests / windows-job (gh)
• Vmas Tests on Linux / unittests (3.9, 12.1) / linux-job (gh)

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[vmoens]

High level feedback: I'd rather name them LSTMCell and GRUCell and rename them during import (to avoid clunky names, we already have so many!)
In the tests I would check that if the parameters of both lstms are the same, the output matches.
We'll need the loop versions too. For these, we should also implement the biderectional feature and more than one level...

[vmoens]

Small edit in the doc but otherwise looks good to me!

[vmoens]

I tested torch.compile and I get

torch._dynamo.exc.Unsupported: TorchDynamo purposely graph breaks on RNN, GRU, LSTMs

so we should consider not inheriting from RNNs in PT, but just from torch.nn.Module.
What are we using from LSTM? Maybe we can do

class LSTM(nn.Module):
    some_method = nn.LSTM.some_method

for the stuff we need to pull from there