Remove and forbid direct inclusion of half.hpp.

[mzient]

Category:

Refactoring (Redesign of existing code that doesn't affect functionality)

Description:

We ship half.hpp from caffe, but we only use it in host code. Usage in device code results in missing includes etc. Moreover, we don't (and shouldn't) use Caffe's half directly.
The inclusion of half.hpp poses problems when an attempt is made to compile a file using half.hpp with NVCC.
This PR removes direct use of half.hpp and adds #error that safeguards against such misuse.

See #5651

Additional information:

Affected modules and functionalities:

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Tests:

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DALI team only

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