Make interface of Dropout backwards compatible

Compiler/ml.py

@@ -1051,7 +1051,14 @@ class Dropout(NoVariableLayer):
     :param shape: list [N, ...] where N is the number of examples and an arbitrary amount of further dimensions
     :param alpha: probability (power of two)
     """
-    def __init__(self, shape, alpha=0.5):
+    def __init__(self, N, d1=None, d2=1, alpha=0.5):
+        if isinstance(N, list) or isinstance(N, tuple):
+            shape = N
+            assert d1 is None, ("If shape is given as list/tuple, d1 must be None. "
+                                "Alpha must be passed explicitly for backwards compatibility.")
+        else:
+            assert d1 is not None, "At least one non-batch dimension must be set"
+            shape = [N, d1] if d2 == 1 else [N, d1, d2]
         self.N = shape[0]
         self.X = Tensor(shape, sfix)
         self.Y = Tensor(shape, sfix)