Add support for half type for the http restful api

[venuswu]

Half type is widely used in the deeplearning inference, but the tf-serving doesn't support half type in the restful api,
I submit a pr to solve this problem, please check.

[google-cla]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[venuswu]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

Googlers: Go here for more info.

@googlebot I signed it!

[netfs]

dont roll your own version for float16 conversion (its hard to get that correct). instead use standard @FP16 library that is used in TF. specifically use fp16_ieee_from_fp32_value() API (from fp16.h), and write the output as decimal number.

example code:
https://github.com/tensorflow/tensorflow/blob/2ba6502de549c20c7498f133792cf3223eabc274/tensorflow/lite/delegates/gpu/common/convert.cc#L303

you can refer to the fp16 library via @FP16 bazel target in the BUILD file.

also your change is incomplete. you need to handle input conversion (json -> tensor) for DT_HALF. please update AddValueToTensor() method.

and finally add unit tests for your code.

[venuswu]

Thank you , I'll try.

[teamwong111]

I think you can adjust the type of input through tf.cast(), so that you don't need to add support for Half type.

[Hanyu-Jin]

Hi, I'm facing the same problem and I'd like to know when this pr will be approved.