Adding mxnet components to rl_coach/architectures (#60)

Adding mxnet components to rl_coach architectures.

- Supports PPO and DQN
- Tested with CartPole_PPO and CarPole_DQN
- Normalizing filters don't work right now (see #49) and are disabled in CartPole_PPO preset
- Checkpointing is disabled for MXNet

rl_coach/architectures/mxnet_components/middlewares/middleware.py

@@ -0,0 +1,61 @@
+from typing import Union
+from types import ModuleType
+
+import mxnet as mx
+from mxnet.gluon import nn
+from rl_coach.architectures.middleware_parameters import MiddlewareParameters
+from rl_coach.architectures.mxnet_components.layers import convert_layer
+from rl_coach.base_parameters import MiddlewareScheme
+
+nd_sym_type = Union[mx.nd.NDArray, mx.sym.Symbol]
+
+
+class Middleware(nn.HybridBlock):
+    def __init__(self, params: MiddlewareParameters):
+        """
+        Middleware is the middle part of the network. It takes the embeddings from the input embedders,
+        after they were aggregated in some method (for example, concatenation) and passes it through a neural network
+        which can be customizable but shared between the heads of the network.
+
+        :param params: parameters object containing batchnorm, activation_function and dropout properties.
+        """
+        super(Middleware, self).__init__()
+        self.scheme = params.scheme
+
+        with self.name_scope():
+            self.net = nn.HybridSequential()
+            if isinstance(self.scheme, MiddlewareScheme):
+                blocks = self.schemes[self.scheme]
+            else:
+                # if scheme is specified directly, convert to MX layer if it's not a callable object
+                # NOTE: if layer object is callable, it must return a gluon block when invoked
+                blocks = [convert_layer(l) for l in self.scheme]
+            for block in blocks:
+                self.net.add(block())
+                if params.batchnorm:
+                    self.net.add(nn.BatchNorm())
+                if params.activation_function:
+                    self.net.add(nn.Activation(params.activation_function))
+                if params.dropout:
+                    self.net.add(nn.Dropout(rate=params.dropout))
+
+    @property
+    def schemes(self) -> dict:
+        """
+        Schemes are the pre-defined network architectures of various depths and complexities that can be used for the
+        Middleware. Should be implemented in child classes, and are used to create Block when Middleware is initialised.
+
+        :return: dictionary of schemes, with key of type MiddlewareScheme enum and value being list of mxnet.gluon.Block.
+        """
+        raise NotImplementedError("Inheriting embedder must define schemes matching its allowed default "
+                                  "configurations.")
+
+    def hybrid_forward(self, F: ModuleType, x: nd_sym_type, *args, **kwargs) -> nd_sym_type:
+        """
+        Used for forward pass through middleware network.
+
+        :param F: backend api, either `mxnet.nd` or `mxnet.sym` (if block has been hybridized).
+        :param x: state embedding, of shape (batch_size, in_channels).
+        :return: state middleware embedding, where shape is (batch_size, channels).
+        """
+        return self.net(x)