[tflite] Add png to tflite evaluation tool
Sorry, I am probably not the person to review it @gbaned from `git blame  tensorflow/lite/tools/evaluation/stages/image_preprocessing_stage.cc`, the file was mainly touched by  @srjoglekar246 and @thaink  Hi @Ferev Can you please review this PR ? Thank you! Hi @gbaned, is there anything I can do to move forward? > Hi @gbaned, is there anything I can do to move forward?

Hi @freedomtan Sorry for the delay. Nothing pending from your end.  Hi @Ferev Can you please review this PR ? Thank you! Hi @Ferev Can you please review this PR ? Thank you! Hi @Ferev Can you please review this PR ? Thank you! Hi @Ferev Can you please review this PR ? Thank you! Hi @Ferev Can you please review this PR ? Thank you! Hi @Ferev Can you please review this PR ? Thank you! Hi @Ferev Can you please review this PR ? Thank you! Hi @Ferev Can you please review this PR ? Thank you!

 Hi @Ferev Can you please review this PR ? Thank you!
Currently, image preprocessing in `tensorflow/lite/tools/evaluation` only supports `jpeg` and `raw` images. This patch add `png` support.

Why we need `png`:
1. raw rgb files are larger
2. the jpeg used in tensorflow doesn't support lossless compression, which is needed for some picture quality models.
3. some datasets uses the png format