docs: Indicate which model-runtime proto SPI rpcs are optional (#50)

#### Motivation

In the model server gRPC service provider interface, implementation of the `predictModelSize` RPC is optional, and `modelSize` doesn't need to be implemented if sizes are always returned in the `loadModel` response.

This is documented in the [modelmesh docs](https://github.com/kserve/modelmesh-serving/blob/main/docs/runtimes/custom_runtimes.md#model-sizing) but not in the comments of the service .proto file.

#### Modifications

Add appropriate wording to doc comments in the `model-runtime.proto` file to convey the fact these rpcs aren't required.

#### Result

Clearer docs

Signed-off-by: Nick Hill <nickhill@us.ibm.com>

Signed-off-by: Nick Hill <nickhill@us.ibm.com>

src/main/proto/current/model-runtime.proto

@@ -43,9 +43,14 @@ service ModelRuntime {
   // Predict size of not-yet-loaded model - must return almost immediately.
   // Should not perform expensive computation or remote lookups.
   // Should be a conservative estimate.
+  // NOTE: Implementation of this RPC is optional.
   rpc predictModelSize (PredictModelSizeRequest) returns (PredictModelSizeResponse) {}
 
-  // Calculate size (memory consumption) of currently-loaded model
+  // Calculate size (memory consumption) of currently-loaded model.
+  // NOTE: Implementation of this RPC is only required if models' size
+  // is not returned in the response to loadModel. If the size computation
+  // takes a nontrivial amount of time, it's better to return from loadModel
+  // immediately and implement this to perform the sizing separately.
   rpc modelSize (ModelSizeRequest) returns (ModelSizeResponse) {}
 
   // Provide basic runtime status and parameters; called only during startup.