Don't evaluate kernel in LazyEvaluatedKernelTensor._unsqueeze_batch (#1813)

Related to meta-pytorch/botorch#971.

Author: valtron

gpytorch/lazy/lazy_evaluated_kernel_tensor.py

@@ -237,7 +237,7 @@ def add_jitter(self, jitter_val=1e-3):
         return self.evaluate_kernel().add_jitter(jitter_val)
 
     def _unsqueeze_batch(self, dim):
-        return self.evaluate_kernel()._unsqueeze_batch(dim)
+        return self[(slice(None),) * dim + (None,)]
 
     @cached(name="kernel_diag")
     def diag(self):