samples: add tensorflow hello world sample train scripts

Adds training scripts to TensorFlow Hello World sample.

Signed-off-by: Lauren Murphy <lauren.murphy@intel.com>

samples/modules/tensorflow/hello_world/train/README.md

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+# Hello World Training
+
+This example shows how to train a 2.5 kB model to generate a `sine` wave.
+
+## Table of contents
+
+-   [Overview](#overview)
+-   [Training](#training)
+-   [Trained Models](#trained-models)
+-   [Model Architecture](#model-architecture)
+
+## Overview
+
+1. Dataset: Data is generated locally in the Jupyter Notebook.
+2. Dataset Type: **Structured Data**
+3. Deep Learning Framework: **TensorFlow 2**
+4. Language: **Python 3.7**
+5. Model Size: **2.5 kB**
+6. Model Category: **Regression**
+
+## Training
+
+Train the model in the cloud using Google Colaboratory or locally using a
+Jupyter Notebook.
+
+<table class="tfo-notebook-buttons" align="left">
+  <td>
+    <a target="_blank" href="https://colab.research.google.com/github/tensorflow/tensorflow/blob/master/tensorflow/lite/micro/examples/hello_world/train/train_hello_world_model.ipynb"><img src="https://www.tensorflow.org/images/colab_logo_32px.png" />Google Colaboratory</a>
+  </td>
+  <td>
+    <a target="_blank" href="https://github.com/tensorflow/tensorflow/blob/master/tensorflow/lite/micro/examples/hello_world/train/train_hello_world_model.ipynb"><img src="https://www.tensorflow.org/images/GitHub-Mark-32px.png" />Jupyter Notebook</a>
+  </td>
+</table>
+
+*Estimated Training Time: 10 minutes.*
+
+
+## Trained Models
+
+Download Link | [hello_world.zip](https://storage.googleapis.com/download.tensorflow.org/models/tflite/micro/hello_world_2020_12_28.zip)
+------------- | ------------------------------------------------------------------------------------------------------------------------
+
+The `models` directory in the above zip file can be generated by following the
+instructions in the [Training](#training) section above. It
+includes the following 3 model files:
+
+| Name | Format | Target Framework | Target Device |
+| :------------- |:-------------|:-------------|-----|
+| `model.pb` | Keras SavedModel | TensorFlow | Large-Scale/Cloud/Servers   |
+| `model.tflite` *(2.5 kB)*  | Integer Only Quantized TFLite Model | TensorFlow Lite | Mobile Devices|
+| `model.cc`  | C Source File | TensorFlow Lite for Microcontrollers | Microcontrollers |
+
+
+## Model Architecture
+
+The final model used to simulate a sine wave is displayed below. It is a
+simple feed forward deep neural network with 2 fully connected layers with
+ReLu activations and a final fully connected output layer with as shown below.
+
+![model_architecture.png](../images/model_architecture.png)
+
+*This image was derived from visualizing the 'model.tflite' file in [Netron](https://github.com/lutzroeder/netron)*
+