SeetaResearch / Dragon

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.