zero.Init() clarification (#880)

* zero.Init() clarification

clarify that if `model.half()` can't fit into gpu memory `zero.Init()` is a must.

this proposal is via @samyam's clarification shared elsewhere.

Thank you.

* style

* add clarity

* style
Co-authored-by: Olatunji Ruwase <olruwase@microsoft.com>

deepspeed/runtime/zero/partition_parameters.py

@@ -279,6 +279,9 @@ class Init(InsertPostInitMethodToModuleSubClasses):
         For example, if a node has 1TB of memory and 8 GPUs, we could fit a trillion
         parameter model with 4 nodes and 32 GPUs.
 
+        Important: If the fp16 weights of the model can't fit onto a single GPU memory
+        this feature must be used.
+
         .. note::
             Initializes ``torch.distributed`` if it has not already been done so.
             See :meth:`deepseed.init_distributed` for more information.