fix error message (#4556)

* fix error messgae

* typo

* add suggestion to fix error

---------

Co-authored-by: Thomas Ziereis <thomas.ziereis@web.de>

tinygrad/nn/optim.py

@@ -23,7 +23,9 @@ class Optimizer:
 
   def step(self): Tensor.realize(*self.schedule_step())
   def schedule_step(self) -> List[Tensor]:
-    assert Tensor.training, "Tensor.training should be set in the optimizer"
+    assert Tensor.training, (
+            f"""Tensor.training={Tensor.training}, Tensor.training must be enabled to use the optimizer.
+                - help: Consider setting Tensor.training=True before calling Optimizer.step().""")
     return self._step()+self.params+self.buffers
   def _step(self) -> List[Tensor]: raise NotImplementedError