Automatic device assignment

[shinh]

Currently, when -d cuda is specified, some heuristics are currently used:

1. all inputs will be allocated in GPU except ones which are directly fed to Reshape op as its output shape.
2. results of Shape op will be stored in host.
3. if the right hand side of Div is a single float value, chainerx::AsScalar is called internally in XCVM's Div (this weird hack is for y / x.shape[0] where x.shape[0] is a batch_size).

These actually work as mitigation for now, but we should design more sophisticated device assignments. The principle would be

1. Shapes should be in CPU
2. Make it possible to run cross-device binary ops when one of their inputs is a scalar.
3. If requirements cannot be satisfied, insert a custom op (say OnikuxDeviceCopy) which explicitly copy data between devices, probably showing a warning to users.

[shinh]

Memo: classify-int64-2 branch in the pre-release repo