add batch_sequence

[CarloLucibello]

Implements batch_sequence.
Similar to batchseq, takes as input a list of N sequences of different lengths and batches them together.

The output is different though:

• batchseq returns a vector of length Lmax with elements with N observations
• batch_sequence return an array of size (..., Lmax, N).

batch_sequence should be more useful in producing an output compatible with Flux v0.16 recurrent layers and attention block.

Close #78

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 40.47619% with 25 lines in your changes missing coverage. Please review.

Project coverage is 82.42%. Comparing base (e705227) to head (2c6b812).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/batch.jl	35.89%	25 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #197      +/-   ##
==========================================
- Coverage   86.05%   82.42%   -3.63%     
==========================================
  Files          14       15       +1     
  Lines         710      717       +7     
==========================================
- Hits          611      591      -20     
- Misses         99      126      +27     

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