added scalar_from_tensor, added support to Subtensor to index by 0-d tensors

tensor.py

@@ -463,9 +463,21 @@ class TensorFromScalar(Op):
     def perform(self):
         self.outputs[0].data = self.inputs[0].data
     def grad(self, (s,), (dt,)):
-        raise NotImplementedError('todo: ScalarFromTensor')
+        return [ScalarFromTensor(dt)]
 tensor_from_scalar = gof.op.constructor(TensorFromScalar)
 
+class ScalarFromTensor(Op):
+    def __init__(self, s, **kwargs):
+        assert isinstance(s, Tensor)
+        Op.__init__(self, **kwargs)
+        self.inputs = [s]
+        self.outputs = [scal.Scalar(s.dtype)]
+    def perform(self):
+        self.outputs[0].data = self.inputs[0].data
+    def grad(self, (s,), (dt,)):
+        return [TensorFromScalar(dt)]
+scalar_from_tensor = gof.op.constructor(ScalarFromTensor)
+
 ##########################
 # Unary Operations
 ##########################
@@ -673,6 +685,8 @@ class Subtensor(Op, Viewer):
         def asidx(i):
             if isinstance(i, int): return scal.constant(i)
             if isinstance(i, scal.Scalar) and ('int' in i.dtype): return i
+            if isinstance(i, Tensor) and ('int' in i.dtype) and (i.ndim == 0):
+                return scalar_from_tensor(i)
             raise TypeError(Subtensor.e_invalid, i)
 
         x = _as_tensor(x)