Merge pull request #32272 from reedwm/docstring_cherrypicks

[r2.0-rc1 CherryPick]: Improve Layer docstrings in regards to autocasting.

tensorflow/python/keras/engine/base_layer.py

@@ -160,17 +160,60 @@ class Layer(module.Module):
   y = layer(x)
   ```
 
-  A layer subclass can prevent its inputs from being autocasted by
+  Currently, only tensors in the first argument to the layer's `call` method are
+  casted. For example:
+
+  ```
+  class MyLayer(tf.keras.layers.Layer):
+    # Bug! `b` will not be casted.
+    def call(self, a, b):
+      return a + 1., b + 1.
+
+  a = tf.constant(1., dtype="float32")
+  b = tf.constant(1., dtype="float32")
+
+  layer = MyLayer(dtype="float64")
+  x, y = layer(a, b)
+  print(x.dtype)  # float64
+  print(y.dtype)  # float32. Not casted since `b` was not passed to first input
+  ```
+
+  It is recommended to accept tensors only in the first argument. This way,
+  all tensors are casted to the layer's dtype. `MyLayer` should therefore be
+  written as:
+
+  ```
+  class MyLayer(tf.keras.layers.Layer):
+    # Now, all tensor inputs will be casted.
+    def call(self, inputs):
+      a, b = inputs
+      return a + 1., b + 1.
+
+  a = tf.constant(1., dtype="float32")
+  b = tf.constant(1., dtype="float32")
+
+  layer = MyLayer(dtype="float64")
+  x, y = layer((a, b))
+  print(x.dtype)  # float64
+  print(y.dtype)  # float64.
+  ```
+
+  In a future minor release, tensors in other arguments may be casted as well.
+
+  Currently, other arguments are not automatically casted for
+  technical reasons, but this may change in a future minor release.
+
+  A layer subclass can prevent its inputs from being autocasted by passing
   `autocast=False` to the layer constructor. For example:
 
   ```
   class MyLayer(tf.keras.layers.Layer):
 
-    def __init__(**kwargs):
+    def __init__(self, **kwargs):
       kwargs['autocast']=False
       super(MyLayer, self).__init__(**kwargs)
 
-    def call(inp):
+    def call(self, inp):
       return inp
 
   x = tf.ones((4, 4, 4, 4), dtype='float64')
@@ -188,8 +231,8 @@ class Layer(module.Module):
 
   ```
   tf.keras.backend.set_floatx('float64')
-  layer1 = tf.keras.layers.Dense(4),
+  layer1 = tf.keras.layers.Dense(4)
-  layer2 = tf.keras.layers.Dense(4),
+  layer2 = tf.keras.layers.Dense(4)
 
   x = tf.ones((4, 4))
   y = layer2(layer1(x))  # Both layers run in float64
@@ -201,18 +244,18 @@ class Layer(module.Module):
   well:
 
   ```
-  layer1 = tf.keras.layers.Dense(4, dtype='float64'),
+  layer1 = tf.keras.layers.Dense(4, dtype='float64')
-  layer2 = tf.keras.layers.Dense(4, dtype='float64),
+  layer2 = tf.keras.layers.Dense(4, dtype='float64')
 
   x = tf.ones((4, 4))
   y = layer2(layer1(x))  # Both layers run in float64
 
   class NestedLayer(tf.keras.layers.Layer):
-    def __init__(**kwargs):
+    def __init__(self, **kwargs):
-      super(MyLayer, self).__init__(**kwargs)
+      super(NestedLayer, self).__init__(**kwargs)
       self.dense = tf.keras.layers.Dense(4, dtype=kwargs.get('dtype'))
 
-    def call(inp):
+    def call(self, inp):
       return self.dense(inp)
 
   layer3 = NestedLayer(dtype='float64')