Make MultiDiscrete a Tuple-like space (#2364)

* Make MultiDiscrete a Tuple-like space

* Update test cases for MultiDiscrete

gym/spaces/multi_discrete.py

@@ -1,5 +1,7 @@
 import numpy as np
+from gym.logger import warn
 from .space import Space
+from .discrete import Discrete
 
 
 class MultiDiscrete(Space):
@@ -24,7 +26,6 @@ class MultiDiscrete(Space):
     """
 
     def __init__(self, nvec, dtype=np.int64):
-
         """
         nvec: vector of counts of each categorical variable
         """
@@ -54,5 +55,19 @@ class MultiDiscrete(Space):
     def __repr__(self):
         return "MultiDiscrete({})".format(self.nvec)
 
+    def __getitem__(self, index):
+        nvec = self.nvec[index]
+        if nvec.ndim == 0:
+            subspace = Discrete(nvec)
+        else:
+            subspace = MultiDiscrete(nvec, self.dtype)
+        subspace.np_random.set_state(self.np_random.get_state())  # for reproducibility
+        return subspace
+
+    def __len__(self):
+        if self.nvec.ndim >= 2:
+            warn("Get length of a multi-dimensional MultiDiscrete space.")
+        return len(self.nvec)
+
     def __eq__(self, other):
         return isinstance(other, MultiDiscrete) and np.all(self.nvec == other.nvec)