Add a better error message when unsupported ops are used for Dimension

This fix is related to 25790. In 25790, `Dimension` with division will
thrown an TypeError:
```
output_dim = input_shape[-1] / 2
...
TypeError: unsupported operand type(s) for /: 'Dimension' and 'int'
```
The error is expected, as Dimension only support `//` (not `/`).

However, for backward compatible reasons, `__div__`(`/`) is actually
implemented (but deprecated). See code.

This issue causes confusion and it has been mentioned multiple times
in GitHub and StackOverflow(https://stackoverflow.com/questions/51692253/typeerror-unsupported-operand-types-for-dimension-and-int)

This fix is an attempt to print a better error message with:
```
TypeError: unsupported operand type(s) for /: 'Dimension' and 'int', please use // instead
```

Note that this PR does not change any current behavior, and have no impact with respect
to backward-compatible `__div__`. It merely adds additional notes in the error message.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

tensorflow/python/framework/tensor_shape.py

@@ -468,6 +468,54 @@ class Dimension(object):
     """
     return self // other
 
+  def __rdiv__(self, other):
+    """Use `__floordiv__` via `x // y` instead.
+
+    This function exists only to have a better error message. Instead of:
+    `TypeError: unsupported operand type(s) for /: 'int' and 'Dimension'`,
+    this function will explicitly call for usage of `//` instead.
+
+    Args:
+      other: Another `Dimension`.
+
+    Raises:
+      TypeError.
+    """
+    raise TypeError("unsupported operand type(s) for /: '{}' and 'Dimension', "
+                    "please use // instead".format(type(other).__name__))
+
+  def __truediv__(self, other):
+    """Use `__floordiv__` via `x // y` instead.
+
+    This function exists only to have a better error message. Instead of:
+    `TypeError: unsupported operand type(s) for /: 'Dimension' and 'int'`,
+    this function will explicitly call for usage of `//` instead.
+
+    Args:
+      other: Another `Dimension`.
+
+    Raises:
+      TypeError.
+    """
+    raise TypeError("unsupported operand type(s) for /: 'Dimension' and '{}', "
+                    "please use // instead".format(type(other).__name__))
+
+  def __rtruediv__(self, other):
+    """Use `__floordiv__` via `x // y` instead.
+
+    This function exists only to have a better error message. Instead of:
+    `TypeError: unsupported operand type(s) for /: 'int' and 'Dimension'`,
+    this function will explicitly call for usage of `//` instead.
+
+    Args:
+      other: Another `Dimension`.
+
+    Raises:
+      TypeError.
+    """
+    raise TypeError("unsupported operand type(s) for /: '{}' and 'Dimension', "
+                    "please use // instead".format(type(other).__name__))
+
   def __mod__(self, other):
     """Returns `self` modulo `other`.