Set start method to fork for MacOs ARM devices (#6218)

* fixing paralell sampling issue on M1 Macs

* Changing set method defaults for arm vs intel Mac

Changing default multiprocessing start method to "fork" for ARM Macs and leaving default "forkserver" for intel macs.

* Fix docstring of the ZeroInflatedPoisson distribution.

The parameterization uses the variable `mu` but the docstring uses
`mu` in some cases and `theta` in other cases. The fix replaces
all `theta` with `mu` to match the argument name for this distribution.

* Fix debug_print of wrong variable in notebook

* Revert "Fix debug_print of wrong variable in notebook"

This reverts commit 726d8d0.

* Revert "Fix docstring of the ZeroInflatedPoisson distribution."

This reverts commit 631d3d4.

* Run pre-commit

Co-authored-by: Carl Scheffler <[email protected]>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: 3 people

pymc/parallel_sampling.py

@@ -406,8 +406,13 @@ def __init__(
 
         if mp_ctx is None or isinstance(mp_ctx, str):
             # Closes issue https://github.com/pymc-devs/pymc/issues/3849
+            # Related issue https://github.com/pymc-devs/pymc/issues/5339
             if platform.system() == "Darwin":
-                mp_ctx = "forkserver"
+                if platform.processor() == "arm":
+                    mp_ctx = "fork"
+                else:
+                    mp_ctx = "forkserver"
+
             mp_ctx = multiprocessing.get_context(mp_ctx)
 
         step_method_pickled = None