IO loaders for training datasets

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

The implementation is split by responsibility:

• 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.

Keeping this file as an umbrella preserves the public import path while avoiding a single large module that mixes CSV tokenization, NumPy header parsing, byte decoding, and tensor conversion.