[tmva][pymva] Deprecate the PyKeras method

The new Keras 3 API broke the existing TMVA `PyKeras` method.

Re-implementing the method for Keras 3 turned out to be difficult, as it
is not possible anymore to disable eager execution and have a decent
speed in evaluating the model for single events. So large-scale
refactoring would be necessary to implement `PyKeras` again with good
performance (see also #15790).

Nowadays, we also have the RBatchGenerator to train Keras models
directly with batches that are provided by ROOT. Therefore, the TMVA
`PyKeras` method is not the only way anymore to train a Keras model with
ROOT data without 3rd party libraries for the IO. That means it's not an
essential feature anymore, and deprecating it will even make the
situation clearer for the user, as there are not two different ways
anymore to train Keras models on ROOT data.

Author: guitargeek

README/ReleaseNotes/v640/index.md

@@ -38,6 +38,7 @@ The following people have contributed to this new version:
 * Comparing C++ `nullptr` objects with `None` in Python now raises a `TypeError`, as announced in the ROOT 6.38 release notes. Use truth-value checks like `if not x` or `x is None` instead.
 * The `TGLIncludes.h` and `TGLWSIncludes.h` that were deprecated in ROOT 6.38 and scheduled for removal are gone now. Please include your required headers like `<GL/gl.h>` or `<GL/glu.h>` directly.
 * The GLEW headers (`GL/eglew.h`, `GL/glew.h`, `GL/glxew.h`, and `GL/wglew.h`) that were installed when building ROOT with `builtin_glew=ON` are no longer installed. This is done because ROOT is moving away from GLEW for loading OpenGL extensions.
+* The TMVA `PyKeras` method is deprecated. It was broken by the API changes in Keras 3, released in November 2023 and part of TensorFlow 2.16 or newer. The `PyKeras` method will be removed in ROOT 6.42 (unless an updated implementation for Keras 3 that matches usage, performance and stability requirements will be found unexpectedly).
 
 ## Build System
 

tmva/pymva/src/MethodPyKeras.cxx

@@ -182,6 +182,10 @@ void MethodPyKeras::InitKeras() {
    // initialize first Keras. This is done only here when class has
    // all state variable set from options or read from XML file
    // Import Keras
+   Log() << kWARNING
+         << "The PyKeras TMVA method was deprecated in ROOT 6.40 and will be removed in ROOT 6.42, since it was broken "
+            "by the API changes in Keras 3, released in November 2023 and part of TensorFlow 2.16 or newer."
+         << Endl;
 
    if (fUseTFKeras)
       Log() << kINFO << "Setting up tf.keras" << Endl;

tmva/tmva/inc/TMVA/Types.h

@@ -100,7 +100,7 @@ namespace TMVA {
          kPyRandomForest ,
          kPyAdaBoost     ,
          kPyGTB          ,
-         kPyKeras        ,
+         kPyKeras, // deprecated, will be removed in ROOT 6.42
          kPyTorch        ,
          kC50            ,
          kRSNNS          ,