Clarify definition of gather function in docs. (pytorch#1652)

Author: gchanan

torch/_torch_docs.py

@@ -1397,9 +1397,9 @@
 
 For a 3-D tensor the output is specified by::
 
-    out[i][j][k] = input[index[i][j][k]][j][k]  # dim=0
-    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
+    out[i][j][k] = input[index[i][j][k]][j][k]  # if dim == 0
+    out[i][j][k] = input[i][index[i][j][k]][k]  # if dim == 1
+    out[i][j][k] = input[i][j][index[i][j][k]]  # if 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})`