[weight/var_grad] Make internal variable as shared_ptr

[weight/var_grad] Make internal variable as shared_ptr

Internal variables in weights/var_grad, namely, the variable and gradient itself
are changed to shared_ptr so that weights can be shared without worrying about
shallow copies.

Also changed the copy constructor to not create new Tensor as copy constructor
of weight will get called and its unnecessary + unintentional overhead.
As weight is just wrapper over tensor, their copy constructors should follow
same behavior as tensor which is to not create new memory.
Added clone as an alternative to create new copy of a given weight.

See also #774 #766

**Self evaluation:**
1. Build test: [x]Passed [ ]Failed [ ]Skipped
2. Run test: [x]Passed [ ]Failed [ ]Skipped

Signed-off-by: Parichay Kapoor <pk.kapoor@samsung.com>