docs(mge): restore autodiff.Function docs

GitOrigin-RevId: cbd84df9168fd9a7a5480ed5edd88d84efa6d18d

imperative/python/megengine/core/autodiff/grad.py

@@ -20,23 +20,6 @@ from .._imperative_rt import core2, ops
 from ..ops.builtin import Elemwise, OpDef, RemoteSend
 from ..ops.special import Const
 
-""" Some notes:
-    1. Initialize the optimizer:
-        for each trainable parameter:
-            call wrt(param, callback)
-        Each parameter tensor will be assciated with a Tracer object saved in Tensor._extra_data
-    2. Tracer has one member: node, which is a VariableNode
-    3. VariableNode has a OpNode member: opnode
-    4. OpNode has four members:
-        a. id
-        b. inputs, which is made of VariableNode
-        c. outputs, which are weakref's to VariableNode
-        d. backward: call back function
-        e. has_grad_fn: call has_grad_fn(opnode, reached) to check grad exist
-        f. backward_allow_noinput: whether backward allow noinput
-
-"""
-
 _grad_count = 0
 _grad_manager_dict = weakref.WeakValueDictionary()
 
@@ -97,6 +80,64 @@ class Grad:
 
 
 class Function(ops.PyOpBase):
+    """
+    Defines a block of operations with customizable differentiation.
+
+    The computation should be defined in ``forward`` method, with gradient
+    computation defined in ``backward`` method.
+
+    Each instance of ``Function`` should be used only once during forwardding.
+
+    Examples:
+
+    .. code-block::
+        class Sigmoid(Function):
+            def forward(self, x):
+                y = 1 / (1 + F.exp(-x))
+                self.y = y
+                return y
+
+            def backward(self, dy):
+                y = self.y
+                return dy * y * (1-y)
+
+    """
+
+    def forward(self, *args, **kwargs):
+        """
+        Applies operations to ``inputs`` and returns results. It must be overriden by all subclasses.
+
+        :param input: input tensors.
+        :return: a tuple of Tensor or a single Tensor.
+
+        .. note::
+
+            This method should return a tuple of Tensor or a single Tensor representing the output
+            of the function.
+        """
+        raise NotImplementedError
+
+    def backward(self, *output_grads):
+        """
+        Compute the gradient of the forward function. It must be overriden by all subclasses.
+
+        :param output_grads: gradients of outputs that are returned by :meth:`forward`.
+
+        .. note::
+
+            In case when some tensors of outputs are not related to loss function, the corresponding
+            values in ``output_grads`` would be ``None``.
+
+        .. note::
+
+            This method should return a tuple which containing the gradients of all inputs, in the same order
+            as the ``inputs`` argument of :meth:`forward` . A ``Tensor`` could be returned
+            instead if there is only one input. If users want to stop the propagation of some gradients,
+            the corresponding returned values should be set ``None`` .
+
+        """
+        raise NotImplementedError
+
     def _default_rule(self, *args):
         ret = self.forward(*args)
         self.__single_output = isinstance(ret, core2.Tensor)