[opt_transport] Replace np.sum(a * b) with np.vdot(a, b) (#475)

* Replace np.sum(a * b) with np.vdot(a, b)

* add @oyamad suggestion to mention the use of vdot

---------

Co-authored-by: mmcky <mamckay@gmail.com>

Author: suda-yuga

lectures/opt_transport.md

@@ -667,10 +667,12 @@ X
 Sure enough, we have the same solution and the same cost
 
 ```{code-cell} ipython3
-total_cost = np.sum(X * C)
+total_cost = np.vdot(X, C)
 total_cost
 ```
 
+Here we use [np.vdot](https://numpy.org/doc/stable/reference/generated/numpy.vdot.html) for the trace inner product of X and C
+
 ### A Larger Application
 
 Now let's try using the same package on a slightly larger application.