[RLlib] Add missing __init__.py to algorithms/dqn folder.

[MaxVanDijck]

Why are these changes needed?

Trying to run the code in:

ray/rllib/algorithms/dqn/dqn.py

Line 497 in a13fc9a

from ray.rllib.algorithms.dqn.torch.dqn_rainbow_torch_rl_module import (

results in

ModuleNotFoundError: No module named 'ray.rllib.algorithms.dqn.torch'

Related issue number

#46773

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[simonsays1980]

LGTM. Great catch! Thanks for the fix @MaxVanDijck!

[sven1977]

LGTM! Thanks @MaxVanDijck

[sven1977]

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