add ad. to Week1_Lecture2.ipynb
add some information at the top of Week1_Lecture2.ipynb
Revert "add some information at the top of Week1_Lecture2.ipynb"

This reverts commit 63bb51b88d231d5da998f8cc167decba08a7cfaf.
add ad. to Week1_Lecture3.ipynb
add ad. to Week2_Lecture4.ipynb
add ad. to Week2_Lecture5.ipynb