Make it easier to apply transforms to vectors of the same size.

**Is your feature request related to a problem? Please describe.**

In general, trainers such as KMeans, expect the feature vector to be a vector of known size. The quickest way to provide this information is in the class that defines the model input schema, you can use the `[VectorType(n)]` attribute where `n` is the size of your vector. Alternatively, you can use `SchemaDefinition` similar to this [sample](https://gist.github.com/luisquintanilla/b1367fd8ba000e455b2e43a23dafa645). In this second approach, you still need to have an object to define the schema of objects.

**Describe the solution you'd like**

For dense vectors where the size is the same, there's two approaches we might consider:

- Automatically detect the vector size.
- Enable users to specify at runtime that a column / property is a vector without using `SchemaDefinition`.

**Describe alternatives you've considered**

- Use DataFrame (Microsoft.Data.Analysis)