Undefined names: missing imports?

[cclauss]

flake8 testing of https://github.com/d2l-ai/d2l-en on Python 3.8.0

$ flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics

./d2l/d2l.py:361:5: F821 undefined name 'DATA_URL'
    DATA_URL+'kaggle_house_pred_train.csv',
    ^
./d2l/d2l.py:515:13: F821 undefined name 'read_time_machine'
    lines = read_time_machine()
            ^
./d2l/d2l.py:761:27: F821 undefined name 'read_data_nmt'
    text = preprocess_nmt(read_data_nmt())
                          ^
./d2l/d2l.py:1481:17: F821 undefined name 'read_ptb'
    sentences = read_ptb()
                ^
./d2l/d2l.py:1590:34: F821 undefined name 'read_data_ml100k'
    data, num_users, num_items = read_data_ml100k()
                                 ^
5     F821 undefined name 'DATA_URL'
5

https://flake8.pycqa.org/en/latest/user/error-codes.html

On the flake8 test selection, this PR does not focus on "style violations" (the majority of flake8 error codes that psf/black can autocorrect). Instead these tests are focus on runtime safety and correctness:

• E9 tests are about Python syntax errors usually raised because flake8 can not build an Abstract Syntax Tree (AST). Often these issues are a sign of unused code or code that has not been ported to Python 3. These would be compile-time errors in a compiled language but in a dynamic language like Python they result in the script halting/crashing on the user.
• F63 tests are usually about the confusion between identity and equality in Python. Use ==/!= to compare str, bytes, and int literals is the classic case. These are areas where a == b is True but a is b is False (or vice versa). Python >= 3.8 will raise SyntaxWarnings on these instances.
• F7 tests logic errors and syntax errors in type hints
• F82 tests are almost always undefined names which are usually a sign of a typo, missing imports, or code that has not been ported to Python 3. These also would be compile-time errors in a compiled language but in Python a NameError is raised which will halt/crash the script on the user.

[astonzhang]

See https://github.com/d2l-ai/d2l-en/issues/685

[cclauss]

One down, four to go.

[Binh0103]

.

[astonzhang]

We have fixed the missing imports. For the remaining flake8 style issues, we'll fix in the end

[cclauss]

$ flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics

./d2l/d2l.py:1654:24: F821 undefined name 'vocab'
    premise = np.array(vocab[premise],
                       ^
./d2l/d2l.py:1656:27: F821 undefined name 'vocab'
    hypothesis = np.array(vocab[hypothesis],
                          ^
2     F821 undefined name 'vocab'
2

[cclauss]

Fixed in #953