generalize batchseq to sequence of generic arrays

[CarloLucibello]

Implement the following behavior for batchseq: Given an input list xs containing B arrays of size * x Li, where is common to all arrays and Li can vary, return a list of length Lmax where the elements have size ` x B`.

Have to depend on NNlib for this, not sure it is worth it.

Also, we deprecate rpad since it is piracy of a Base function in favor of a more generic rpad_constant (mimicking the NNlib naming scheme).

Fix #118

[codecov-commenter]

Codecov Report

Merging #126 (bd3bef8) into main (a85c098) will increase coverage by 0.17%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #126      +/-   ##
==========================================
+ Coverage   88.77%   88.94%   +0.17%     
==========================================
  Files          13       13              
  Lines         561      570       +9     
==========================================
+ Hits          498      507       +9     
  Misses         63       63              

Impacted Files	Coverage Δ
src/utils.jl	90.90% <100.00%> (+0.66%)	:arrow_up:

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