[tmva][pymva] Deprecate the PyKeras method

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

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          ,