Refactoring for the following external data format support.

Signed-off-by: yoco <yoco@skymizer.com>

- Parse initializer starts with "=[" for external data
- Print initializer starts with "=[" for external data
- Add unit test

Signed-off-by: yoco <yoco@skymizer.com>

The string should use double quote, not single quote.

Signed-off-by: yoco <yoco@skymizer.com>

Signed-off-by: yoco <yoco@skymizer.com>

Signed-off-by: yoco <yoco@skymizer.com>

We cannot reference the onnx:: namespace directly
because it is compile time defined (users can choose to
call it differently).

Signed-off-by: yoco <yoco@skymizer.com>

Signed-off-by: yoco <yoco@skymizer.com>

- While print, check if the external data format is specified before the
  internall data. Theoratically they are mutually exclusize, however,
  during some conversion internal state, the external and internal might
  be specified at the same time. In that case, we want to print the
  external data format.
- Replace Parse with PARSE macro, apply check automatically.
- Update document, add the definition of str-str.

Signed-off-by: yoco <yoco@skymizer.com>

Signed-off-by: Justin Chu <justinchuby@users.noreply.github.com>