Update MPI Estimator to support Pytorch IPEX training (#8303)

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docs/readthedocs/source/doc/Orca/Overview/distributed-training-inference.md

@@ -304,3 +304,43 @@ result_shards = est.predict(shards)
 The input to `predict` methods can be an *XShards*, or a *numpy array*. See the *data-parallel processing pipeline* [page](./data-parallel-processing.html) for more details.
 
 View the related [Python API doc](https://bigdl.readthedocs.io/en/latest/doc/PythonAPI/Orca/orca.html#orca-learn-openvino-estimator) for more details.
+
+### 7. MPI Estimator
+The Orca MPI Estimator is to run distributed training job based on MPI.
+
+#### Preparation:
+* Configure password-less ssh from the master node (the one you'll launch training from) to all other nodes.
+
+* All hosts have the same working directory.
+* All hosts have the same Python environment in the same location.
+
+#### Train
+Then the user may create a MPI Estimator as follows:
+```python
+from bigdl.orca.learn.mpi import MPIEstimator
+
+est = MPIEstimator(model_creator=model_creator,
+                   optimizer_creator=optimizer_creator,
+                   loss_creator=None,
+                   metrics=None,
+                   scheduler_creator=None,
+                   config=config,
+                   init_func=init,  # Init the distributed environment for MPI if any
+                   hosts=hosts,
+                   workers_per_node=workers_per_node,
+                   env=None)                   
+```
+Then the user can perform distributed model training as follows:
+```python
+# read spark Dataframe
+df = spark.read.parquet("data.parquet")
+
+# distributed model training
+est.fit(data=df, epochs=1, batch_size=4, feature_col="feature", label_cols="label")
+
+```
+The input to `fit` methods can be an Spark Dataframe, or a callable  function to return a `torch.utils.data.DataLoader`. 
+
+View the related [Python API doc](https://bigdl.readthedocs.io/en/latest/doc/PythonAPI/Orca/orca.html#orca-learn-mpi-mpi-estimator) for more details.
+
+

docs/readthedocs/source/doc/PythonAPI/Orca/orca.rst

@@ -77,3 +77,11 @@ orca.learn.openvino.estimator
     :members:
     :undoc-members:
     :show-inheritance:
+
+orca.learn.mpi.mpi_estimator
+------------------------------
+
+.. autoclass:: bigdl.orca.learn.mpi.MPIEstimator
+    :members:
+    :undoc-members:
+    :show-inheritance: