Vectorization for temperature simulation
Besides the hints in comments, I would like to add the following:


Replace the abbreviated variables by useful names. This will increase the performance of your code reviewer.
Try to keep all your comments in English (see 1.).
Preallocate variables if you really want to write their value inside a loop. Even in C++ you would not write Ta(i,t)=Tinitial;.
Do you really need all N*M*num_t values? If not, you should only store the current and the last iteration.
Now try to begin to replace the repetitive statements like Ta(i,t)=T(i,t0); and the sourounding for-loop with a single statement. You don't need to replace all at once.
Split your code into a main function and one or more subfunctions. A recent Matlab itself may then increase execution speed of the loops.
You calculate c=k/Inc_x1; very often but neither k nor Inc_x1 changes. Try to identify lines which can be computed outside of the loop. Maybe you need to introduce more variables (with good names!).
Replace the while-loop by a for-loop. It will be more readable.
Correct the indentation of your code. Press Ctrl-A and Ctrl-I.
If you have a 2D object, then why not use a 2D array for this. If you really need the results for all times, then you should have a 3D-array. You may then initialize it with temperature = zeros(M, N, T_max);


If you made all this changes, feel free to present your new readable and beautiful code.