Arm backend: Example external model to be used by the ahead of time arm compiler (#10810)

Example of an external model to be loaded as a module by the
aot_arm_compiler.py.
Useful for documentation.

Author: Juanfi8

examples/arm/example_modules/README.md

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+# Example of an external model for the ARM AOT Compiler
+Example of an external Python file to be used as a module by the `run.sh` (and the `aot_arm_compiler.py`) scripts in `examples/arm` directory. 
+Just pass the path of the `add.py` file as `--model_name`:
+
+`ModelUnderTest` should be a `torch.nn.module` instance.
+
+`ModelInputs` should be a tuple of inputs to the forward function.

examples/arm/example_modules/add.py

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+import torch
+
+
+class myModelAdd(torch.nn.Module):
+    def __init__(self):
+        super().__init__()
+
+    def forward(self, x):
+        return x + x
+
+
+ModelUnderTest = myModelAdd()
+ModelInputs = (torch.ones(5),)