Add size/dimensionality documentation for torch.gather. (pytorch#1645)

Author: gchanan

torch/_torch_docs.py

@@ -1401,6 +1401,12 @@
  out[i][j][k] = input[i][index[i][j][k]][k] # dim=1
  out[i][j][k] = input[i][j][index[i][j][k]] # dim=2
 
+If :attr:`input` is an n-dimensional tensor with size
+:math:`(x_0, x_1..., x_{i-1}, x_i, x_{i+1}, ..., x_{n-1})`
+and :attr:`dim` = i, then :attr:`index` must be an n-dimensional tensor with size
+:math:`(x_0, x_1, ..., x_{i-1}, y, x_{i+1}, ..., x_{n-1})` where y >= 1 and
+:attr:`out` will have the same size as :attr:`index`.
+
 Args:
  input (Tensor): The source tensor
  dim (int): The axis along which to index