fix GPU sign...these tests aren't very good

tinygrad/ops_gpu.py

@@ -67,7 +67,7 @@ class Exp(Function):
 class Sign(Function):
   @staticmethod
   def forward(ctx, input):
-    return unary_op(ctx, 'a >= 0', input)
+    return unary_op(ctx, '(a > 0) - (a < 0)', input)
 
   @staticmethod
   def backward(ctx, grad_output):

tinygrad/tensor.py

@@ -274,7 +274,7 @@ class Tensor:
     return self * (self.softplus().tanh()) # x*tanh(softplus(x))
 
   def abs(self):
-    return self.relu() + (-1.0*self).relu()
+    return self * self.sign()
 
   def _pool2d(self, py, px):
     xup = self[:, :, :self.shape[2]-self.shape[2]%py, :self.shape[3]-self.shape[3]%px]