Update Infer proto docs to mention BFloat16 type

[rmccorm4]

What this PR does / why we need it:

Adds mention of BFloat16 (BF16) type to the Infer API, similarly handled as FP16. Triton Inference Server is adding TYPE_BF16 support and we want to keep parity with the KServe docs: triton-inference-server/common#49

[aws-kf-ci-bot]

Hi @rmccorm4. Thanks for your PR.

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[deadeyegoodwin]

@yuzisun This looks good to me.

[yuzisun]

/ok-to-test

[yuzisun]

/lgtm
/approve

[kserve-oss-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rmccorm4, yuzisun

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