Adding initialstates function to RNNs

[MartinuzziFrancesco]

Adding initialstates function as per discussion in #2514

PR Checklist

• Tests are added
• Entry in NEWS.md
• Documentation, if applicable

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.81%. Comparing base (74c3a63) to head (48402d7).
Report is 5 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2541       +/-   ##
===========================================
+ Coverage   32.69%   65.81%   +33.12%     
===========================================
  Files          34       35        +1     
  Lines        1976     2030       +54     
===========================================
+ Hits          646     1336      +690     
+ Misses       1330      694      -636     

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[CarloLucibello]

It should be defined also for RNN, LSTM, ....

[MartinuzziFrancesco]

It should be defined also for RNN, LSTM, ....

Ah sorry I was meaning to ask, I'll go forward with that

[CarloLucibello]

Should test that the Flux.initialstates(rnn) returns what is supposed to return for all layers.