TensorFlow's high-level API for model training, evaluation, and prediction, simplifying distributed ML without managing computational graphs or sessions
Estimators represent a complete model but also look intuitive enough to less user. The Estimator API provides methods to train the model, to judge the model’s accuracy, and to generate predictions. TensorFlow provides a programming stack consisting of multiple API layers like in the below image:

There are two types of estimators; you can either choose the pre-made Estimators, alternatively, you can write your own custom Estimators. Estimators-based models can be run on local hosts as well as on distributed multi-server environment without changing your model. In addition, you can run Estimators-based models on CPUs, GPUs, or TPUs without having to record your model.
On top of that, the Estimator includes default behavior common to training jobs, such as saving and restoring checkpoints, creating summaries, etc. An Estimator will require you to write a model_fn and an input_fn that correspond to the model and input portions of your TensorFlow graph.
TensorFlow's Estimator API offers two types of Estimators: pre-made Estimators that come ready to use, and custom Estimators that you write yourself. Both types represent a complete model and can be run on local hosts or in a distributed multi-server environment without changing the underlying model. This flexibility lets you choose a ready-made solution for common tasks or build a custom Estimator when your use case requires it.
An Estimator requires you to write a model_fn and an input_fn that correspond to the model and input portions of your TensorFlow graph. The model_fn defines the model itself, while the input_fn handles the data that feeds into it. Estimators are themselves built on tf.keras.layers, which makes customizing these functions easier.
Yes, Estimator-based models can run on CPUs, GPUs, or TPUs without having to rewrite your model. They can also move between a local host and a distributed multi-server environment without changing the model itself. This portability is one of the core design goals of the Estimator API.
Estimators encapsulate four main features: training, evaluation, prediction, and exporting your model for serving. Training runs the model on a given input for a fixed number of steps, evaluation checks the model against a test set, and prediction runs inference using the trained model. On top of these features, the Estimator includes default behavior common to training jobs, such as saving and restoring checkpoints and creating summaries.
Estimators are generally easier to use than low-level TensorFlow APIs because they let you develop a model with high-level, intuitive code while the Estimator builds the graph for you. They also simplify sharing implementations between model developers. In addition, Estimators provide a safely distributed training loop that controls building the graph, initializing variables, loading data, handling exceptions, creating checkpoint files, recovering from failures, and saving summaries for TensorBoard.
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