Add a stub for host offloading docs (#8656)

Author: tengyifei

examples/host_offloading/README.md

@@ -1,3 +1,19 @@
-This directory will contain a self-contained example for host offloading
-by the time of the 2.6 release.
+## Host offloading example
 
+When doing reverse-mode automatic differentiation, many tensors are saved
+during the forward pass to be used to compute the gradient during the backward pass.
+Previously you could use `torch_xla.utils.checkpoint` to discard tensors that's easy
+to recompute later, called "checkpointing" or "rematerialization". Now PyTorch/XLA
+also supports a technique called "host offloading", i.e. moving the tensor to host
+and moving them back, adding another tool in the arsenal to save memory. Use
+`torch_xla.experimental.stablehlo_custom_call.place_to_host` to move a tensor to host
+and `torch_xla.experimental.stablehlo_custom_call.place_to_device` to move a tensor
+back to the device. For example, you can use this to move intermediate activations
+to host during a forward pass, and move those activations back to device during
+the corresponding backward pass.
+
+Because the XLA graph compiler aggressively reorders operations, host offloading is
+best used in combination with `scan`.
+
+TODO(yifeit): Clean up the example in https://github.com/tengyifei/playground/blob/master/graph_transforms/offloading.py
+and put that here.