Fix using CuPy for eager mode

[esmeetu]

Currently vLLM inference on multi-node is broken after #2811 introduced cupy.
This PR support multi-node inference for eager mode.
Likely temporarily fix #2826 #2959 with eager mode.

[WoosukKwon]

I think the right way to do this is to correctly set up CuPy in multi-node setting? WDYT?

[Yard1]

Considering CuPy is supposed to just be a workaround for now, I think it makes more sense to avoid using it unless necessary (and it should not be necessary in eager mode).

That being said, it does have to work in multi-node setting to enable CUDA graphs there. It should be relatively straightforward to set it up.

[esmeetu]

I think the right way to do this is to correctly set up CuPy in multi-node setting? WDYT?

@WoosukKwon I understand in #2811 that you just fix memory leak issue on cuda graph by introducing CuPy. And eager mode doesn't have that issue, so we can keep it as it is before we found some benefit.

[WoosukKwon]

@esmeetu Yep. Makes sense! Let's merge this PR and figure out how to correctly set up CuPy in the multi-node setting.