Rework module structure and packages

[juliabeliaeva]

Current module structure has some problems:

1. "dataset" module contains two separate groups of classes: preprocessing api and implementation and dataset api and implementation.
2. "dataset" is not an essential module, and "api" should not have a dependency on it.
3. Some api classes, like TensorShape or Operation, would fit better in the "api" module and not in "dataset" module.

To fix these problems, this PR introduces new module structure:

1. "api" has only api classes (including preprocessing api), and no platform-specific code.
2. "impl" contains implementations, like ImageRecognitionModelBase, preprocessing implementations (including platform-dependent), utility classes, etc. It depends on "api" module.
3. "dataset" has dataset api and implementations. Depends on "api" and "impl" modules.

This PR also changes some packages, including moving "onnx" module classes to the package org.jetbrains.kotlinx.dl.onnx, and ModelHub and related classes from keras.loaders to just loaders (maybe here other package name like modelhub or hub could be better?). Packages in "tensorflow" module are unchanged, but we may need to refactor them in the future releases.

[ermolenkodev]

It looks like some tests are not updated, at least these two:

impl/src/jvmTest/kotlin/org/jetbrains/kotlinx/dl/impl/preprocessing/PreprocessingTensorTest.kt
tensorflow/src/test/kotlin/org/jetbrains/kotlinx/dl/api/core/integration/SavedModelTest.kt

[ermolenkodev]

Not related to the changes, but in this version, I cannot build the project using the command line ./gradlew, only in IntelliJ Idea. Do you know how to use the same toolchain as in IDE when building through the command line?

[ermolenkodev]

Can you please elaborate on what should go to the impl package in the new structure? Implementation of interfaces defined in any other package?

[juliabeliaeva]

Not related to the changes, but in this version, I cannot build the project using the command line ./gradlew, only in IntelliJ Idea. Do you know how to use the same toolchain as in IDE when building through the command line?

No, that was my mistake. I fixed it and it should work now, thank you for pointing it out.

[juliabeliaeva]

Can you please elaborate on what should go to the impl package in the new structure? Implementation of interfaces defined in any other package?

Implementations of interfaces from the "api" package, platform-specific code, additional utilities such as CocoUtils or ImageNetPreprocessing, etc.

Initially, I wanted to extract "preprocessing" module, but I noticed that a lot of classes also do not fit into the "api" category and also should be moved into a separate module.