QuantizationMode should be algebraic instead of scalar #285

Previously MLX supported affine quantization only, it was fine to have groupSize and bits as separate arguments of quantization methods. However, the newly supported mxfp4 only allows groupSize = 32 and bits = 4, it'd be better to confine quantization modes with a full sum type, e.g.

enum QuantizationMode: Equatable {
  case affine(groupSize: Int, bits: Int)
  case mxfp4
}

If, in the future, MLX adds support to more quantization modes (e.g. mxfpN), mxfp4 can be converted into a convenience method to maintain source compatibility, i.e.

enum QuantizationMode: Equatable {
  case affine(groupSize: Int, bits: Int)
  case mxfpN(n: Int) 
}

extension QuantizationMode {
  static let mxfp4: Self = .mxfpN(n: 4)
}

mode == .mxfp4 // still works because of the Equatable conformance

It will be a source breaking change for all use sites of QuantizationMode though, but I believe the use of sum type in this case will make the API more extensible and fool-proof.