fix: resolve TensorFlow keras lazy loader recursion error

Set TF_USE_LEGACY_KERAS=1 to force TensorFlow 2.19.0 to use
tf_keras instead of Keras 3.x if the environment has Keras 3.x
installed. This work around prevents TensorFlow's lazy loader
from infinitely recursing when trying to determine which
Keras version to use. See:

    https://keras.io/getting_started/#tensorflow--keras-2-backwards-compatibility.

This fixes generate_test_models.py which was failing with:
"RecursionError: maximum recursion depth exceeded in comparison"
at line checking "if self._tfll_keras_version == 'keras_3'"

BUG=#3125

Author: rkuester

tensorflow/lite/micro/testing/BUILD

@@ -83,6 +83,7 @@ py_library(
     deps = [
         requirement("numpy"),
         requirement("tensorflow"),
+        requirement("tf_keras"),
     ],
 )
 

tensorflow/lite/micro/testing/generate_test_models.py

@@ -26,6 +26,14 @@
 from __future__ import division
 from __future__ import print_function
 
+import os
+# Workaround for TensorFlow 2.19.0 keras lazy loader issue. If Keras 3.0 is
+# installed, TensorFlow's lazy loader infinitely recurses when trying to
+# determine which Keras version to use. This forces TensorFlow to use the
+# one provided by package tf_keras. This must be set before importing
+# `tensorflow`. See https://keras.io/getting_started/#tensorflow--keras-2-backwards-compatibility.
+os.environ['TF_USE_LEGACY_KERAS'] = '1'
+
 from absl import app
 import numpy as np
 import tensorflow as tf

third_party/python_requirements.in

@@ -28,6 +28,7 @@
 
 bitarray
 tensorflow
+tf_keras
 twine
 numpy
 mako

third_party/python_requirements.txt

@@ -1034,7 +1034,9 @@ tensorflow==2.18.0 \
     --hash=sha256:bfdd65ea7e064064283dd78d529dd621257ee617218f63681935fd15817c6286 \
     --hash=sha256:ec4133a215c59314e929e7cbe914579d3afbc7874d9fa924873ee633fe4f71d0 \
     --hash=sha256:f8c946df1cb384504578fac1c199a95322373b8e04abd88aa8ae01301df469ea
-    # via -r third_party/python_requirements.in
+    # via
+    #   -r third_party/python_requirements.in
+    #   tf-keras
 tensorflow-io-gcs-filesystem==0.37.1 \
     --hash=sha256:0df00891669390078a003cedbdd3b8e645c718b111917535fa1d7725e95cdb95 \
     --hash=sha256:249c12b830165841411ba71e08215d0e94277a49c551e6dd5d72aab54fe5491b \
@@ -1057,6 +1059,10 @@ termcolor==2.5.0 \
     --hash=sha256:37b17b5fc1e604945c2642c872a3764b5d547a48009871aea3edd3afa180afb8 \
     --hash=sha256:998d8d27da6d48442e8e1f016119076b690d962507531df4890fcd2db2ef8a6f
     # via tensorflow
+tf-keras==2.18.0 \
+    --hash=sha256:c431d04027eef790fcd3261cf7fdf93eb74f3cb32e05078b57b7f5a54bd53262 \
+    --hash=sha256:ebf744519b322afead33086a2aba872245473294affd40973694f3eb7c7ad77d
+    # via -r third_party/python_requirements.in
 tomli==2.0.2 \
     --hash=sha256:2ebe24485c53d303f690b0ec092806a085f07af5a5aa1464f3931eec36caaa38 \
     --hash=sha256:d46d457a85337051c36524bc5349dd91b1877838e2979ac5ced3e710ed8a60ed