Update the direct TensorFlow dependency to ~=2.4.

This means that we would accept a hot fix and a new minor version , but not accept major version changes without releasing TFP.

PiperOrigin-RevId: 428811702

requirements.txt

@@ -8,6 +8,6 @@ pandas~=1.1.4
 scipy~=1.2.1
 sklearn~=1.0.2
 tensorflow-datasets~=4.5.2
-tensorflow-estimator>=2.3.0
+tensorflow-estimator~=2.4
 tensorflow-probability~=0.15.0
-tensorflow>=1.14
+tensorflow~=2.4

setup.py

@@ -36,13 +36,8 @@ setup(
         'scipy~=1.2.1',
         'sklearn~=1.0.2',
         'tensorflow-datasets~=4.5.2',
-        'tensorflow-estimator>=2.3.0',
+        'tensorflow-estimator~=2.4',
         'tensorflow-probability~=0.15.0',
+        'tensorflow~=2.4',
     ],
-    # Explicit dependence on TensorFlow is not supported.
-    # See https://github.com/tensorflow/tensorflow/issues/7166
-    extras_require={
-        'tf': ['tensorflow>=1.0.0'],
-        'tf_gpu': ['tensorflow-gpu>=1.0.0'],
-    },
     packages=find_packages())