Infer GRW steps from shape

[canyon289]

@ricardoV94 Thanks for adding this commit to the grw branch.

I've moved it to another PR solely for reasons of my own learning. and not because I disagree with the changes or think they shouldn't be merged.. Once the GRW pr is good enough as is I'd like to go through and "relearn" everything about it so I can make sure I understood that scope. I'll then do the same for this logic too!

Hopefully that makes sense

[codecov]

Codecov Report

Merging #5541 (47571a7) into grw (2171790) will increase coverage by 0.09%.
The diff coverage is 28.57%.

@@            Coverage Diff             @@
##              grw    #5541      +/-   ##
==========================================
+ Coverage   86.36%   86.46%   +0.09%     
==========================================
  Files          81       81              
  Lines       14261    14437     +176     
==========================================
+ Hits        12317    12483     +166     
- Misses       1944     1954      +10     

Impacted Files	Coverage Δ
pymc/distributions/timeseries.py	27.93% <28.57%> (-0.64%)	⬇️
pymc/sampling_jax.py	96.89% <0.00%> (-1.53%)	⬇️
pymc/parallel_sampling.py	86.71% <0.00%> (-1.00%)	⬇️
pymc/distributions/discrete.py	99.66% <0.00%> (-0.11%)	⬇️
pymc/model.py	84.82% <0.00%> (ø)

[canyon289]

Ill come back this this one now that GRW is merged
#5298