Turn on Python formatting checks as part of CI. (#316)

* Groundwork to enable python format checks (via yapf) as part of the CI.

With this change:
 * we are set up to use yapf as part of the docker container
 * pigweed patch is updated to use the google Python style (pep8 with indent of 2 spaces)
 * some documentation updates.

The code still needs to be formatted, and then the Python formatting check will be turned on as part of ci.

BUG=http://b/194404216

* Format all the code.

Once this change is merged, we will turn on Python formatting check as
part of the CI.

* minor formatting update.

* Turn on Python formatting checks as part of CI.

Also, add an option to fix the formatting via the test_code_style.sh
script.

BUG=http://b/194404984 and http://b/194404216

CONTRIBUTING.md

@@ -221,6 +221,14 @@ Below are some tips that might be useful and improve the development experience.
 
 * Get a copy of [cpplint](https://github.com/google/styleguide/tree/gh-pages/cpplint)
 
+* [yapf](https://github.com/google/yapf/) should be used for formatting Python
+  code. For example:
+
+  ```
+  pip3 install yapf
+  yapf log_parser.py -i --style='{based_on_style: pep8, indent_width: 2}'
+  ```
+
 * Add a git hook to check for code style etc. prior to creating a pull request:
   ```
   cp tensorflow/lite/micro/tools/dev_setup/pre-push.tflm .git/hooks/pre-push
@@ -338,18 +346,8 @@ Below are some tips that might be useful and improve the development experience.
 
 ## Python notes
 
-Most PRs for TensorFlow Lite Micro will be C++ only. Adding some notes on Python
-that can be expanded and improved as necessary.
-
 *   [TensorFlow guide](https://www.tensorflow.org/community/contribute/code_style#python_style)
     for Python development
 
-*   [yapf](https://github.com/google/yapf/) should be used for formatting. For example:
-
-    ```
-    pip3 install yapf
-    yapf log_parser.py -i --style='{based_on_style: pep8, indent_width: 2}'
-    ```
-
 # Continuous Integration System
   * Some [additional documentation](docs/continuous_integration.md) on the TFLM CI.

tensorflow/lite/micro/tools/ci_build/test_code_style.sh

@@ -62,20 +62,28 @@ LICENSE_CHECK_RESULT=$?
 ############################################################
 # Formatting Check
 ############################################################
-# We are currently ignoring Python files (with yapf as the formatter) because
-# that needs additional setup.  We are also ignoring the markdown files to allow
-# for a more gradual rollout of this presubmit check.
+
+if [[ ${1} == "--fix_formatting" ]]
+then
+  FIX_FORMAT_OPTIONS="--fix"
+else
+  FIX_FORMAT_OPTIONS=""
+fi
+
 micro/tools/make/downloads/pigweed/pw_presubmit/py/pw_presubmit/format_code.py \
-  kernels/internal/reference/ \
-  micro/ \
-  ../../third_party/ \
+  ${FIX_FORMAT_OPTIONS} \
+  -e c/common.c \
+  -e core/api/error_reporter.cc \
   -e kernels/internal/reference/integer_ops/ \
   -e kernels/internal/reference/reference_ops.h \
+  -e kernels/internal/types.h \
+  -e experimental \
+  -e schema/schema_generated.h \
+  -e schema/schema_utils.h \
   -e "\.inc" \
-  -e "\.md" \
-  -e "\.py"
+  -e "\.md"
 
-CLANG_FORMAT_RESULT=$?
+FORMAT_RESULT=$?
 
 #############################################################################
 # Avoided specific-code snippets for TFLM
@@ -121,8 +129,12 @@ popd
 # Re-enable exit on error now that we are done with the temporary git repo.
 set -e
 
+if [[ ${FORMAT_RESULT}  != 0 ]]
+then
+  echo "The formatting errors can be fixed with tensorflow/lite/micro/tols/ci_build/test_code_style.sh --fix_formatting"
+fi
 if [[ ${LICENSE_CHECK_RESULT}  != 0 || \
-      ${CLANG_FORMAT_RESULT}   != 0 || \
+      ${FORMAT_RESULT}         != 0 || \
       ${GTEST_RESULT}          != 0 || \
       ${ERROR_REPORTER_RESULT} != 0 || \
       ${ASSERT_RESULT}         != 0    \