improve: move training dependencies into extras

Author: samedii

README.rst

@@ -66,5 +66,8 @@ Install lantern from pypi using pip or poetry:
 
 .. code-block::
 
-    poetry add pytorch-lantern
-    # pip install pytorch-lantern
+    poetry add pytorch-lantern[training]
+
+.. code-block::
+
+    pip install pytorch-lantern[training]

lantern/__init__.py

@@ -1,8 +1,8 @@
 from lantern.numpy_seed import numpy_seed
 from lantern.star import star
 from lantern.functional_base import FunctionalBase
-from lantern.tensor import Tensor
-from lantern.numpy import Numpy
+from pytorch_types import Tensor
+from pytorch_types import Numpy
 from lantern.epochs import Epochs
 from lantern.metric import Metric
 from lantern.metric_table import MetricTable