I have a DataFrame df like this:

Pattern    String                                       
101        hi, how are you?
104        what are you doing?
108        Python is good to learn.

I want to create ngrams for String Column. I've create unigram using split() and stack()

new= df.String.str.split(expand=True).stack()

However, I want to create ngrams (bi, tri, quad etc)

Do a little preprocessing on your text column, and then a little shifting + concatenation:

# generate unigrams 
unigrams  = (df['String'].str.lower().str.replace(r'[^a-z\s]', '').str.split(expand=True).stack())# generate bigrams by concatenating unigram columns
bigrams = unigrams + ' ' + unigrams.shift(-1)
# generate trigrams by concatenating unigram and bigram columns
trigrams = bigrams + ' ' + unigrams.shift(-2)# concatenate all series vertically, and remove NaNs
pd.concat([unigrams, bigrams, trigrams]).dropna().reset_index(drop=True)

0                   hi
1                  how
2                  are
3                  you
4                 what
5                  are
6                  you
7                doing
8               python
9                   is
10                good
11                  to
12               learn
13              hi how
14             how are
15             are you
16            you what
17            what are
18             are you
19           you doing
20        doing python
21           python is
22             is good
23             good to
24            to learn
25          hi how are
26         how are you
27        are you what
28        you what are
29        what are you
30       are you doing
31    you doing python
32     doing python is
33      python is good
34          is good to
35       good to learn
dtype: object