Docs: add a document that describes the testing of the compiler.

Author: nadavrot

docs/Testing.md

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+## Testing the Glow compiler
+
+The Glow test suite contains four major categories: unit tests, regression
+tests, example programs, and the model loader.  Unit tests are the small tests
+that stress specific parts of the compiler.  These tests are added to the
+compiler when developing a feature. For example, we train a number of small
+network and perform a gradient check on the operators.  We also compile networks
+to IR and look for specific patterns.  Regression tests are tests that are added
+when we fix bugs.  Both regression tests and feature tests are found under the
+"test/" directory. To run the feature and regression tests run "ninja test".
+
+## Example test suites.
+
+We rely on external test suites to test the compiler. We use the data sets
+CIFAR10 and MNIST (located in the "example/" directory) to test the correctness
+of the whole system.  The script under 'utils/' download and extract the data
+set.
+
+## Model Loader
+
+We test the correctness of the Glow implementation by loading Caffe2 models and
+executing them end-to-end. The program 'loader' loads model, a png file, and
+runs a single pass of inference. If everything goes right the output of the
+program is identical to the output of the Caffe2 model. Unfortunately, the caffe
+model does not describe what the input format should be. Should the pixels be
+between zero and one, or negative 128 to positive 128? The user needs to be
+aware of these things when running the models. The script in the directory
+'utils/' downloads a number of pre-trained networks that we can use for testing.
+