Fix thrust failure when transfering data from device_vector to host_vector with vectors of size 1

[ttnghia]

This is a fix for the problem that popped up in the bug #6364.

I debug this problem and discovered that thrust crashes only when the vector size is 1. There should not be any problem with the vector size, thus the bug should be due to something else. Finally, I found a fix for this: adding qualifiers __host__ __device__ to the type transformer.

It really weird that without those qualifiers, there is runtime problem only when the vector size is 1.

[vuule]

@ttnghia can you add a test for this fix?

[codecov]

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[ttnghia]

@ttnghia can you add a test for this fix?

Sure. I've added that.

[harrism]

I think we could name things a bit better while we are at it, and I'm not sure about that test.

[ttnghia]

Thanks for the comments. I have made a couple of changes.

[harrism]

BTW, In my own unrelated work I caused some tests to fail and observed these CUDA errors (not crashes) as well. Adding the code from this PR fixed the errors and allowed me to see the differences. Let's get this merged ASAP!

[codereport]

lgtm 👍, copyright needs to be updated in both files though

[codereport]

👍

[ttnghia]

lgtm 👍, copyright needs to be updated in both files though

Thanks for reminding me about that. I have updated.

[harrism]

@gpucibot merge