Make TensorColumn shape and dtype properties lazy but memoized (#257)

This improves performance in cases where a lot of `TensorColumns` get created but the shapes and dtypes aren't used (like in the dataloaders.)

merlin/dtypes/__init__.py

@@ -19,6 +19,7 @@
 from merlin.dtypes.aliases import *
 from merlin.dtypes.base import DType
 from merlin.dtypes.registry import _dtype_registry
+from merlin.dtypes.shape import Dimension, Shape
 
 # Convenience alias for registering dtypes
 register = _dtype_registry.register

merlin/table/tensor_column.py

@@ -19,7 +19,7 @@
 
 import merlin.dtypes as md
 from merlin.dispatch.lazy import lazy_singledispatch
-from merlin.dtypes.shape import Shape
+from merlin.dtypes import DType, Shape
 
 
 class Device(Enum):
@@ -67,12 +67,10 @@ def __init__(self, values: Any, offsets: Any = None, dtype=None, _ref=None, _dev
 
         self._values = values
         self._offsets = offsets
-
-        shape = self._construct_shape(values, offsets)
-        self._dtype = md.dtype(dtype or values.dtype).with_shape(shape)
-
+        self._dtype = dtype or values.dtype
         self._ref = _ref
         self._device = _device
+        self._shape = None
 
     def cpu(self):
         """
@@ -104,7 +102,9 @@ def gpu(self):
 
     @property
     def shape(self) -> Shape:
-        return self._dtype.shape
+        if not self._shape:
+            self._shape = self._construct_shape(self.values, self.offsets)
+        return self._shape
 
     @property
     def is_list(self) -> Shape:
@@ -128,6 +128,8 @@ def offsets(self):
 
     @property
     def dtype(self):
+        if not isinstance(self._dtype, DType):
+            self._dtype = md.dtype(self._dtype).with_shape(self.shape)
         return self._dtype
 
     def __len__(self):