message: Fix warning caused by keep_dims in sparse_grad.py
  While running tests, noticed the following warning:
  ```
  ...
  tensorflow/python/ops/sparse_grad.py:281: calling sparse_reduce_sum (from tensorflow.python.ops.sparse_ops) with keep_dims is deprecated and will be removed in a future version.
  Instructions for updating:
  keep_dims is deprecated, use keepdims instead
  ```
  This fix fixes the warning.
diff: @@ -278,7 +278,7 @@ def _SparseSoftmaxGrad(op, grad):
  indices, sp_output.values * sp_grad.values, shape)
  # [..., B, 1], dense.
  - sum_reduced = -sparse_ops.sparse_reduce_sum(sp_product, [-1], keep_dims=True)
  + sum_reduced = -sparse_ops.sparse_reduce_sum(sp_product, [-1], keepdims=True)
  # sparse [..., B, C] + dense [..., B, 1] with broadcast; outputs sparse.
  sp_sum = sparse_ops.sparse_dense_cwise_add(sp_grad, sum_reduced)
  