Make fully Bayesian imports lazy (#3265)

[sdaulton]

Summary:
X-link: facebook/Ax#5148

import botorch eagerly loaded Pyro via two import paths:

1. botorch.fit → from pyro.infer.mcmc import MCMC, NUTS
2. botorch.models.__init__ → fully_bayesian.py → Pyro imports

This diff makes fully Bayesian imports lazy so that import botorch no longer
eagerly loads Pyro. The imports are deferred until fit_fully_bayesian_model_nuts
is called or SaasFullyBayesianSingleTaskGP/SaasFullyBayesianMultiTaskGP are
accessed from botorch.models.

Reviewed By: saitcakmak

Differential Revision: D99687047

[meta-codesync]

@sdaulton has exported this pull request. If you are a Meta employee, you can view the originating Diff in D99687047.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.98%. Comparing base (0c93a38) to head (072d5c0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3265   +/-   ##
=======================================
  Coverage   99.98%   99.98%           
=======================================
  Files         221      221           
  Lines       21916    21924    +8     
=======================================
+ Hits        21912    21920    +8     
  Misses          4        4           

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