refactor!: support only Multibackend Keras.

Also:
 * Add types
 * Improve metrics tracking file.

keras_tuner/backend/__init__.py

@@ -1,25 +0,0 @@
-# Copyright 2019 The KerasTuner Authors
-#
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-#     https://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-"""Keras backend module.
-
-This module adds a temporarily Keras API surface that is fully under KerasTuner
-control. This allows us to switch between Keras 3 and `tf.keras`, as well
-as add shims to support older version of `tf.keras`.
-
-- `config`: check which backend is being run.
-- `keras`: The full `keras` API (via `keras` 3 or `tf.keras`).
-- `ops`: `keras.ops`, always tf backed if using `tf.keras`.
-"""
-
-from keras_tuner.backend import config, io, keras, ops, random

keras_tuner/backend/io.py

@@ -13,49 +13,57 @@
 # limitations under the License.
 
 
-import glob as built_in_glob
-import os
 import shutil
+from pathlib import Path
+from typing import cast
 
-from keras_tuner.backend import config
+from _typeshed import OpenTextMode
+from keras.api.backend import backend
 
-if config.backend() == "tensorflow":
+if backend() == "tensorflow":
     import tensorflow as tf
 else:
     tf = None
 
+# the tf options are for non-locking thread.
 
-def exists(path):
+
+def exists(path: str) -> bool:
+    """Check if file exists at path."""
     if tf is None:
-        return os.path.exists(path)
+        return Path(path).exists()
     return tf.io.gfile.exists(path)
 
 
-def rmtree(path):
+def rmtree(path: str) -> None:
+    """Remove the whole directory tree under path."""
     if tf is None:
         return shutil.rmtree(path)
     return tf.io.gfile.rmtree(path)
 
 
-def File(filename, mode):
+def File(filename: str, mode: OpenTextMode):
     if tf is None:
-        file = open(filename, mode)
+        file = Path(filename).open(mode)
     else:
         file = tf.io.gfile.GFile(filename, mode)
 
     return file
 
 
-def glob(path):
+def glob(path: str) -> list[str]:
+    """Return a list globs from pattern."""
     if tf is None:
-        return built_in_glob.glob(path)
+        return cast(list[str], list(Path().glob(path)))
     return tf.io.gfile.glob(path)
 
 
-def makedirs(path):
+def makedirs(path: str) -> None:
+    """Create directry and parents at path, unless it exists."""
     if tf is None:
-        return os.makedirs(path, exist_ok=True)
-    return tf.io.gfile.makedirs(path)
+        Path.mkdir(Path(path), exist_ok=True)
+    else:
+        tf.io.gfile.makedirs(path)
 
 
 # The following code is forked from Keras: