Prune python tensor (#5596)

* move tensor to c++

Signed-off-by: daquexian <daquexian566@gmail.com>

* align with master and fix export error

Signed-off-by: daquexian <daquexian566@gmail.com>

* reformat

Signed-off-by: daquexian <daquexian566@gmail.com>

* rename some functions

Signed-off-by: daquexian <daquexian566@gmail.com>

* refine

Signed-off-by: daquexian <daquexian566@gmail.com>

* remove unused header

Signed-off-by: daquexian <daquexian566@gmail.com>

* restore device repr and str

Signed-off-by: daquexian <daquexian566@gmail.com>

* upgrade pybind11 to 2.7.0 for https://github.com/pybind/pybind11/pull/2956



Signed-off-by: daquexian <daquexian566@gmail.com>

* impl parameter in c++, remove determine() call

Signed-off-by: daquexian <daquexian566@gmail.com>

* let parameter derive from TensorIf to own a separate grad_fn_node, add missing get_device method, remove out-dated methods in docs

Signed-off-by: daquexian <daquexian566@gmail.com>

* add test_parameter.py, fix requires_grad_ and get_de...