IO loaders for training datasets

NN.Runtime.Autograd.Train.IoLoader is the public umbrella for file-backed training loaders.

The loader surface has three parts:

• IoLoader.Common contains small shared parser utilities and safety limits.
• IoLoader.Csv contains CSV-to-tensor dataset readers.
• IoLoader.Npy contains the supported NumPy .npy subset for vectors and matrices.

This umbrella keeps the import path stable while the parsing code stays close to the file format it checks.