message: Change name of `enable_xla_sharding` field of TPUCompileMetadata proto `xla_sharding_config` and make the field to be a enum.
diff: syntax = "proto3";
  -option cc_enable_arenas = true;
  -
  package tensorflow.tpu;
  import "tensorflow/compiler/xla/xla.proto";
  @@ -10,6 +8,8 @@ import "tensorflow/core/framework/tensor_shape.proto";
  import "tensorflow/core/framework/types.proto";
  import "tensorflow/core/protobuf/tpu/dynamic_padding.proto";
  +option cc_enable_arenas = true;
  +
  // This is an experimental proto used in the TF/XLA bridge to store metadata to
  // a compile op (e.g. _TPUCompileMlir).
  // TODO(lyandy): Deprecate proto once generic metadata proto is created.
  @@ -35,9 +35,15 @@ message TPUCompileMetadataProto {
  // Whether this argument will receive the same data across all replicas.
  bool is_same_data_across_replicas = 5;
  + enum EnableXlaSharding {
  + DISALLOWED = 0;
  + // Sharding is allowed if host training loop exists.
  + TENTATIVE = 1;
  + ALLOWED = 2;
  + }
  // Whether to allow XLA to produce separate programs to shard/unshard this
  // argument. Requires this arg to be an on-device variable.
  - bool enable_xla_sharding = 6;
  + EnableXlaSharding enable_xla_sharding = 6;
  }
  repeated Arg args = 1;
  