Improve type casters

[oleksandr-pavlyk]

Improved efficiency of pybind11 classes and type casters.

Type casters in pybind11 generated with PYBIND11_TYPE_CASTER macro rely on default constructor of C++ type. The default constructed value is then overwritten by load method. C++ types such as sycl::device and sycl::queue do non-trivial amount of work in default constructor. This work was ranking high in certain workloads (such as example/pybind11/onemkl_gemv/sycl_timing_solver.py).

Same applies to auto-generated type-casters for classes dpctl::memory::usm_memory and dpctl::tensor::usm_ndarray.

This PR introduces DPCTL_TYPE_CASTER macro that defines unique_ptr to the C++ type, rather than the type itself. This avoids superfluous call to default constructor altogether.

Instead of redefining pyobject_caster automatically generated for dpctl::tensor::usm_ndarray and dpctl::memory::usm_memory a singleton class dpctl::detail::dpctl_api is added that creates objects used by default constructors of these classes.

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpctl/pulls/837/index.html

[coveralls]

Coverage decreased (-0.008%) to 82.818% when pulling ba50f98 on improve-type-casters into 50be964 on master.

[oleksandr-pavlyk]

On my development Gen9 box this change speeds up the test suite run-time from 85 seconds to 54 seconds.

Now:

======================== 3107 passed, 173 skipped, 1 xfailed in 53.81s ===============================

Before:

====================== 3107 passed, 173 skipped, 1 xfailed in 84.72s (0:01:24) =======================

[github-actions]

Deleted rendered PR docs from intelpython.github.com/dpctl, latest should be updated shortly. 🤞