Add MLContext.Model.Load overload that takes a file path

[eerhardt]

Today, the only way to load a model is by using a Stream:

machinelearning/src/Microsoft.ML.Data/Model/ModelOperationsCatalog.cs

Lines 36 to 41 in e2464f6

	/// <summary>
	/// Load the model from the stream.
	/// </summary>
	/// <param name="stream">A readable, seekable stream to load from.</param>
	/// <returns>The loaded model.</returns>
	public ITransformer Load(Stream stream) => TransformerChain.LoadFrom(_env, stream);

We should add an overload that takes a string filePath as a convenience API over top of this. That way all the callers don't need to open a FileStream and dispose of it, etc, just to load a model.

@TomFinley @CESARDELATORRE @sfilipi @shauheen

[codemzs]

You have my full support!

[CESARDELATORRE]

And of course you have my full support, too. :)
I opened an issue related to this topic in Nov. 2018:

#1689

This change will make the code a lot cleaner!
:)

[glebuk]

Sounds great. Also todo in this issue:

• Ensure to have a file overload for both load and save model methods
• Update samples to use the overload when loading/saving from file.