added warnings to unimplemented methods (#1317)

* added warnings and removed default optimizer

* opt

* Apply suggestions from code review

Co-authored-by: Jirka Borovec <Borda@users.noreply.github.com>

docs/source/introduction_guide.rst

@@ -269,7 +269,6 @@ In PyTorch we do it as follows:
 
 
 In Lightning we do the same but organize it under the configure_optimizers method.
-If you don't define this, Lightning will automatically use `Adam(self.parameters(), lr=1e-3)`.
 
 .. code-block:: python
 
@@ -278,6 +277,17 @@ If you don't define this, Lightning will automatically use `Adam(self.parameters
       def configure_optimizers(self):
         return Adam(self.parameters(), lr=1e-3)
 
+.. note:: The LightningModule itself has the parameters, so pass in self.parameters()
+
+However, if you have multiple optimizers use the matching parameters
+
+.. code-block:: python
+
+    class LitMNIST(pl.LightningModule):
+
+        def configure_optimizers(self):
+            return Adam(self.generator(), lr=1e-3), Adam(self.discriminator(), lr=1e-3)
+
 Training step
 ^^^^^^^^^^^^^