[Breaking Change] remove NewtonGirardAdditiveKernel

- The functionality of this kernels has not disappeared, but this
  kernel cannot work without the last_dim_is_batch_option.
- The examples/00_Basic_Usage/kernels_with_additive_or_product_structure.ipynb
  notebook describes how to replicate the functionality of this kernel
  using the gpytorch.utils.sum_interaction_terms utility.

Author: gpleiss

gpytorch/kernels/__init__.py

@@ -18,7 +18,6 @@
 from .matern_kernel import MaternKernel
 from .multi_device_kernel import MultiDeviceKernel
 from .multitask_kernel import MultitaskKernel
-from .newton_girard_additive_kernel import NewtonGirardAdditiveKernel
 from .periodic_kernel import PeriodicKernel
 from .piecewise_polynomial_kernel import PiecewisePolynomialKernel
 from .polynomial_kernel import PolynomialKernel
@@ -52,7 +51,6 @@
     "LinearKernel",
     "MaternKernel",
     "MultitaskKernel",
-    "NewtonGirardAdditiveKernel",
     "PeriodicKernel",
     "PiecewisePolynomialKernel",
     "PolynomialKernel",