Remove remaining references to class_weight='auto'

[jnothman]

class_weight='auto' was deprecated in #4347, with 'balanced' as an alternative. However it appears that there are a few residual uses of it in documentation:

• doc/modules/grid_search.rst
• sklearn/svm/classes.py

Clean them up.

[krishnakalyan3]

@jnothman I found following occurrences of auto

sklearn/svm/classes.py
line 591,594

591     class_weight : {dict, 'auto'}, optional
592         Set the parameter C of class i to class_weight[i]*C for
593         SVC. If not given, all classes are supposed to have
594         weight one. The 'auto' mode uses the values of y to
595         automatically adjust weights inversely proportional to
596         class frequencies.

doc/modules/grid_search.rst
line 104,

103   [{'C': scipy.stats.expon(scale=100), 'gamma': scipy.stats.expon(scale=.1),
104     'kernel': ['rbf'], 'class_weight':['auto', None]}]

and will replace auto occurrences with balanced.

[krishnakalyan3]

Sorry you are right. Will fix it. Thanks