Pass num_warmup as kwarg to step_warmup

[penelopeysm]

cf. TuringLang/Turing.jl#2678

Sometimes, step_warmup() needs to know how many warmup steps it's doing. This is the case for adaptive Hamiltonians, which call AdvancedHMC.adapt!, which has different behaviour depending on whether it's the first warmup step, last warmup step, or something in between (see here).

This PR therefore exposes this information to step_warmup (previously, the num_warmup kwarg would have been 'swallowed' by mcmcsample).

[github-actions]

AbstractMCMC.jl documentation for PR #178 is available at:
https://TuringLang.github.io/AbstractMCMC.jl/previews/PR178/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (ae9760d) to head (fdab17c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

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##   main   #178   +/-   ##
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