SeetaResearch / Dragon

Summary:
This commit fixes the issue on building with CUDA11 and CUDNN8.
Besides, C++14 is enabled by default instead of C++11 to support CUB 1.9+,
and for this reason, the compiler is required to be gcc5/clang6/msvc141 or higher.

Summary:
This commit uses sphinx to generate C++ API documentation
whose style and theme are consistent with the Python API.

Summary:
This commit computes the correct device for an existing output tensor.

Summary:
This commit enforces all the vm packages to take api/core structure
to adapt to the more complex future developments.

Summary:
This commit correctly passes the extra arguments
when executing a compiled graph function.

Summary:
This commit enforces the shape and data type inherited from the same metaclass,
which ensures the consistency between different styles.

Summary:
This commit normalizes the inconsistent math notations in docstring.

Summary:
This commit aligns the properties and methods of tensor class.

Summary:
This commit adds the clear method to free resources manually
if the workspace instance is referenced circularly.

Summary:
This commit moves the workspace api into the current workspace instance.
For this reason, the namespace ``dragon.workspace`` is removed for simplicity.

Summary:
This commit tests the executing of native ops and verifies the results.
Several bugs are found and fixed according to these tests.

Summary:
This commit removes the redundant workspace reference
when executing a tensor operation.

Summary:
We forgot to handle the inplace case that generated
the random elements on the output(i.e. the input).

Besides, this commit also fixes the omitted `RunOnDevice` for cudnn activations,
which will rightly dispatches the implementation.