Add readme for call a model (lost due to merge)

experimental/torch_xla2/README.md

@@ -60,3 +60,96 @@ pip install -e .[tpu] -f https://storage.googleapis.com/libtpu-releases/index.ht
 pip install -r test-requirements.txt
 pytest test
 ```
+
+## Run a model
+
+Now let's execute a model under torch_xla2. We'll start with a simple 2-layer model
+it can be in theory any instance of `torch.nn.Module`.
+
+```python
+import torch
+import torch.nn as nn
+import torch.nn.functional as F
+
+
+class MyModel(nn.Module):
+    def __init__(self):
+        super().__init__()
+        self.fc1 = nn.Linear(28 * 28, 120)
+        self.fc2 = nn.Linear(120, 84)
+        self.fc3 = nn.Linear(84, 10)
+
+    def forward(self, x):
+        x = x.view(-1, 28 * 28)
+        x = F.relu(self.fc1(x))
+        x = F.relu(self.fc2(x))
+        x = self.fc3(x)
+        return x
+
+m = MyModel()
+
+# Execute this model using torch
+inputs = torch.randn(3, 3, 28, 28)
+print(m(inputs))
+```
+
+This model `m` contains 2 parts: the weights that is stored inside of the model
+and it's submodules (`nn.Linear`).
+
+To execute this model with `torch_xla2`; we need to move the tensors involved in compute
+to `XLA` devices. This can be accomplished with `torch_xla2.tensor.move_to_device`.
+
+We need move both the weights and the input to xla devices:
+
+```python
+import torch_xla2
+from torch.utils import _pytree as pytree
+from torch_xla2.tensor import move_to_device
+
+inputs = move_to_device(inputs)
+new_state_dict = pytree.tree_map_only(torch.Tensor, move_to_device, m.state_dict())
+m.load_state_dict(new_state_dict, assign=True)
+
+res = m(inputs)
+
+print(type(res))  # outputs XLATensor2
+```
+
+### Executing with jax.jit
+
+The above script will execute the model using eager mode Jax as backend. This 
+does allow executing torch models on TPU, but is often slower than what we can 
+achieve with `jax.jit`.
+
+`jax.jit` is a function that takes a Jax function (i.e. a function that takes jax array
+and returns jax array) into the same function, but faster.
+
+We have made the `jax_jit` decorator that would accomplish the same with functions
+that takes and returns `torch.Tensor`. To use this, the first step is to create
+a functional version of this model: this means the parameters should be passed in
+as input instead of being attributes on class:
+
+
+```python
+
+def model_func(param, inputs):
+  return torch.func.functional_call(m, param, inputs)
+
+```
+Here we use [torch.func.functional_call](https://pytorch.org/docs/stable/generated/torch.func.functional_call.html) 
+from PyTorch to replace the model
+weights with `param`, then call the model. This is equivalent to:
+
+```python
+def model_func(param, inputs):
+  m.load_state_dict(param)
+  return m(*inputs)
+```
+
+Now, we can apply `jax_jit`
+
+```python
+from torch_xla2.extra import jax_jit
+model_func_jitted = jax_jit(model_func)
+print(model_func_jitted(new_state_dict, inputs))
+```