remove unused copy likelihood state_dict in SingleTaskGP (#194)

Summary:
Pull Request resolved: #194

see title

Reviewed By: danielrjiang

Differential Revision: D16036033

fbshipit-source-id: e85c15f08de9d79e68ad3e5557191164a8e66a1e

Author: sdaulton

botorch/models/converter.py

@@ -62,7 +62,7 @@ def _check_compatibility(models: ModelListGP) -> None:
         )
 
     # TODO: Add support for custom likelihoods
-    if any(hasattr(m, "_likelihood_state_dict") for m in models):
+    if any(getattr(m, "_is_custom_likelihood", False) for m in models):
         raise NotImplementedError(
             "Conversion of models with custom likelihoods is currently unsupported."
         )

botorch/models/gp_regression.py

@@ -6,7 +6,6 @@
 Gaussian Process Regression models based on GPyTorch models.
 """
 
-from copy import deepcopy
 from typing import Any, Optional
 
 import torch
@@ -88,7 +87,7 @@ def __init__(
                 ),
             )
         else:
-            self._likelihood_state_dict = deepcopy(likelihood.state_dict())
+            self._is_custom_likelihood = True
         ExactGP.__init__(self, train_X, train_Y, likelihood)
         self.mean_module = ConstantMean(batch_shape=self._aug_batch_shape)
         self.covar_module = ScaleKernel(