fix: deep copy nuts_sampler_kwarg to prevent .pop side effects (#7652)

pymc/sampling/mcmc.py

@@ -338,6 +338,7 @@ def _sample_external_nuts(
                 UserWarning,
             )
         compile_kwargs = {}
+        nuts_sampler_kwargs = nuts_sampler_kwargs.copy()
         for kwarg in ("backend", "gradient_backend"):
             if kwarg in nuts_sampler_kwargs:
                 compile_kwargs[kwarg] = nuts_sampler_kwargs.pop(kwarg)