[TF 2.4] TF C-API breaking changes at 2.4

[TF 2.4] TF C-API breaking changes at 2.4

Because of breaking changes of TF 2.4
https://github.com/tensorflow/tensorflow/blob/e43be76009614be88454d2fdf2fe702acc5bab77/RELEASE.md#breaking-changes-1
, we need to use different C-API usages for TensorFlow.

The corresponding change is
> C-API functions TF_StringDecode, TF_StringEncode, and TF_StringEncodedSize are no longer relevant and have been removed; see core/platform/ctstring.h for string access/modification in C.

Fixing #3081

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>