Avoid relative imports; import only modules

[justinchuby]

1. Even though relative imports are prevalent in ort, they are confusing, harder to manage and refactor. We should prefer absolute imports for clarity and robustness.
   https://softwareengineering.stackexchange.com/questions/159503/whats-wrong-with-relative-imports-in-python
   https://google.github.io/styleguide/pyguide.html#22-imports
2. Import only modules (in most cases that is not __init__).
   Import only the module instead of classes and functions to (1) keep the namespace clean and provide readers with more context on where its members come from, and (2) prevent circular import errors.

   • Prevent circular import errors: Programming FAQ — Python 3.10.4 documentation
     Circular imports are fine where both modules use the “import ” form of import. They fail when the 2nd module wants to grab a name out of the first (“from module import name”) and the import is at the top level. That’s because names in the 1st are not yet available, because the first module is busy importing the 2nd.

   • Clean namespace: For example, readers don’t need to backtrack to see sleep is a function from time, as opposed to a function defined in the file. https://google.github.io/styleguide/pyguide.html#22-imports