[Bug] SumBatchLazyTensor size is inconsistent with indices
# 🐛 Bug

I want to do a KroneckerProductLazyTensor on a batch of lazyTensor `x` times, then SumBatchLazyTensor and then get a specific row and finally evaluate. The code works if I first do an evaluation on the `sum_a` then retrieve the row (which is inefficient) but gives `size is inconsistent with indices` error if I retrieve the row first and then wants to evaluate.

Interestingly, If I use the same number for the dimension -1 and -2, there would be no error then.

## To reproduce

** Code snippet to reproduce **
```python
import gpytorch
x = 3
a = torch.rand((x, 5, 2, 3))
lazy_a = gpytorch.lazy.NonLazyTensor(a)
assert lazy_a.shape == torch.Size([3, 5, 2, 3])
prod_a = gpytorch.lazy.KroneckerProductLazyTensor(*lazy_a)
assert prod_a.shape == torch.Size([5, 8, 27])
sum_a = gpytorch.lazy.SumBatchLazyTensor(prod_a)
assert sum_a.shape == torch.Size([8, 27])
assert sum_a.evaluate()[0].shape == torch.Size([27])
assert sum_a[0].evaluate().shape == torch.Size([27]) # gives error in here
```

** Stack trace/error message **
```
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-251-7cce10ce99d3> in <module>()
      8 assert sum_a.shape == torch.Size([8, 27])
      9 assert sum_a.evaluate()[0].shape == torch.Size([27])
---> 10 assert sum_a[0].evaluate().shape == torch.Size([27])
     11 

9 frames
/usr/local/lib/python3.6/dist-packages/gpytorch/lazy/lazy_tensor.py in __getitem__(self, index)
   1703         # with the appropriate shape
   1704         if (squeeze_row or squeeze_col or row_col_are_absorbed):
-> 1705             res = delazify(res)
   1706         if squeeze_row:
   1707             res = res.squeeze(-2)

/usr/local/lib/python3.6/dist-packages/gpytorch/lazy/lazy_tensor.py in delazify(obj)
   1753         return obj
   1754     elif isinstance(obj, LazyTensor):
-> 1755         return obj.evaluate()
   1756     else:
   1757         raise TypeError("object of class {} cannot be made into a Tensor".format(obj.__class__.__name__))

/usr/local/lib/python3.6/dist-packages/gpytorch/utils/memoize.py in g(self, *args, **kwargs)
     32         cache_name = name if name is not None else method
     33         if not is_in_cache(self, cache_name):
---> 34             add_to_cache(self, cache_name, method(self, *args, **kwargs))
     35         return get_from_cache(self, cache_name)
     36 

/usr/local/lib/python3.6/dist-packages/gpytorch/lazy/lazy_tensor.py in evaluate(self)
    858             eye = torch.eye(num_rows, dtype=self.dtype, device=self.device)
    859             eye = eye.expand(*self.batch_shape, num_rows, num_rows)
--> 860             res = self.transpose(-1, -2).matmul(eye).transpose(-1, -2).contiguous()
    861         else:
    862             eye = torch.eye(num_cols, dtype=self.dtype, device=self.device)

/usr/local/lib/python3.6/dist-packages/gpytorch/lazy/lazy_tensor.py in matmul(self, other)
   1093 
   1094         func = Matmul()
-> 1095         return func.apply(self.representation_tree(), other, *self.representation())
   1096 
   1097     @property

/usr/local/lib/python3.6/dist-packages/gpytorch/functions/_matmul.py in forward(ctx, representation_tree, rhs, *matrix_args)
     18 
     19         lazy_tsr = ctx.representation_tree(*matrix_args)
---> 20         res = lazy_tsr._matmul(rhs)
     21 
     22         to_save = [orig_rhs] + list(matrix_args)

/usr/local/lib/python3.6/dist-packages/gpytorch/lazy/block_lazy_tensor.py in _matmul(self, rhs)
     64 
     65         rhs = self._add_batch_dim(rhs)
---> 66         res = self.base_lazy_tensor._matmul(rhs)
     67         res = self._remove_batch_dim(res)
     68 

/usr/local/lib/python3.6/dist-packages/gpytorch/lazy/interpolated_lazy_tensor.py in _matmul(self, rhs)
    157     def _matmul(self, rhs):
    158         # Get sparse tensor representations of left/right interp matrices
--> 159         left_interp_t = self._sparse_left_interp_t(self.left_interp_indices, self.left_interp_values)
    160         right_interp_t = self._sparse_right_interp_t(self.right_interp_indices, self.right_interp_values)
    161 

/usr/local/lib/python3.6/dist-packages/gpytorch/lazy/interpolated_lazy_tensor.py in _sparse_left_interp_t(self, left_interp_indices_tensor, left_interp_values_tensor)
    309 
    310         left_interp_t = sparse.make_sparse_from_indices_and_values(
--> 311             left_interp_indices_tensor, left_interp_values_tensor, self.base_lazy_tensor.size()[-1]
    312         )
    313         self._left_interp_indices_memo = left_interp_indices_tensor

/usr/local/lib/python3.6/dist-packages/gpytorch/utils/sparse.py in make_sparse_from_indices_and_values(interp_indices, interp_values, num_rows)
     59     else:
     60         cls = getattr(torch.sparse, type_name)
---> 61     res = cls(index_tensor, value_tensor, interp_size)
     62 
     63     # Wrap things as a variable, if necessary

RuntimeError: size is inconsistent with indices: for dim 1, size is 8 but found index 26
```

## Expected Behavior

Expected to pass the tests.

## System information

**Please complete the following information:**
- GPyTorch Version 0.3.5
- PyTorch Version 1.2.0
- Ubuntu 18.04.3 LTS


diff --git a/gpytorch/lazy/sum_batch_lazy_tensor.py b/gpytorch/lazy/sum_batch_lazy_tensor.py
index 7d9f738c..171b3837 100644
--- a/gpytorch/lazy/sum_batch_lazy_tensor.py
+++ b/gpytorch/lazy/sum_batch_lazy_tensor.py
@@ -54,3 +54,6 @@ class SumBatchLazyTensor(BlockLazyTensor):
     def diag(self):
         diag = self.base_lazy_tensor.diag().sum(-2)
         return diag
+
+    def evaluate(self):
+        return self.base_lazy_tensor.evaluate().sum(dim=-3)  # BlockLazyTensors always use dim3 for the block_dim
['gpytorch/lazy/sum_batch_lazy_tensor.py']