[Breaking] Redesign broadcasts/reductions to enable multi axis reductions

[coreylowman]

Progress on #163
Progress on #191

Breaking Changes

• Add trait Broadcast and Broadcast::broadcast()
  • Remove trait Broadcast1, trait Broadcast2, trait Broadcast3, trait Broadcast4
  • Remove .broadcast2(), .broadcast2(), .broadcast3(), .broadcast4()
• Add trait Reduce
  • Remove trait Reduce1

Description

Turned out to be really annoying to implement with the previous approach.

Summary:

• Add private BroadcastRef and BroadcastMut
• Add private IndexRef and IndexMut
• Add Accumulator
• Combine device broadcast & reduce into 1 trait DeviceReduce
• Combine tensor broadcast & reduce into 2 traits Broadcast and Reduce

Broadcast and reduce are very much opposites of each other. They share the exact same behavior but with the direction reversed. The new method works by always broadcasting the smaller array to a bigger one using the new BroadcastRef/Mut. By implementing ElementRef/Mut for both the BroadcastRef/Mut structs and raw arrays, you can just treat broadcast/reduce as a normal element-wise operation.