Fix prediction slowdown from eager kernel evaluation

[kayween]

This fixes the test-time slowdown for batched inputs as reported in #2736

Root Cause

Note that .evaluate_kernel is called for test covariances in ExactGP._get_test_prior_mean_and_covariances. This would materialize the K(test, test) matrix explicitly (e.g., RBF and Matern kernels), even in cases when only predictive variances are needed.

gpytorch/gpytorch/models/exact_gp.py

Lines 426 to 427 in ee0564b

	test_test_covar = joint_covar[..., num_train:, num_train:].evaluate_kernel()
	test_train_covar = joint_covar[..., num_train:, :num_train].evaluate_kernel()

The Fix

Remove .evaluate_kernel calls in ExactGP. The prediction strategies should call .evaluated_kernel when needed.

[kayween]

This PR moves .evaluate_kernel calls from ExactGP to prediction strategies. In particular, it changes _get_test_prior_mean_and_covariances, which LatentKroneckerGP overrides.

@SebastianAment Would this cause any friction on the BoTorch side?