Fix bfloat16 serialization of Tensors.

Previously, Python serialization and deserialization used the half_val field of TensorProto, whereas C++ serialization used the int_val field. However, C++ bfloat16 deserialization was always broken, so it was never possible to correctly deserialize a bfloat16 Tensor.

The only reason serialization worked at all was because of the generic tensor_contents bytes serialization.

PiperOrigin-RevId: 178966536