Fix model serialization with external data in current directory (#17311)
When original model has external data in current directory, saving the
optimized model will raise File not found exception during looking for
external data file under root directory "/". This fix will look under
current directory for this case.
I manually tested an extra case and it is working: Original model with
external data in root directory ("/"), and save optimized to current
directory.
BTW, there is another bug found: when
"session.optimized_model_external_initializers_min_size_in_bytes" is set
a large value, some tensor is still pointed to the original external
data file. Add a TODO in unit test for this bug. Possible solution: load
external data into memory before saving model.