Add typing annotations

import numpy as np

from collections import defaultdict

from deepchem.utils.typing import PymatgenComposition

from deepchem.utils.typing import PymatgenComposition, DefaultDict

from deepchem.feat import MaterialCompositionFeaturizer

elements_tl = [

  >>> features = featurizer.featurize([comp])

  """

  def get_fractions(self, comp):

  def get_fractions(self, comp: DefaultDict) -> np.ndarray:

    if all(e in elements_tl for e in comp):

      return np.array([comp[e] if e in comp else 0 for e in elements_tl],

                      np.float32)

    else:

      return None

  def parse_fractions(self, form):

  def parse_fractions(self, form: str) -> str:

    while '/' in form:

      di = form.index('/')

      num1 = [x for x in re.findall(r'\d*\.*\d*', form[:di]) if x != ''][-1]

      form = form[:di - len(num1)] + fract + form[di + len(num2) + 1:]

    return form

  def parse_formula(self, formula):

  def parse_formula(self, formula: str) -> DefaultDict:

    stack = []

    curr_str = ''

    i = 0

"""Type annotations that are widely used in DeepChem"""

from typing import Any, Callable, List, Sequence, Tuple, TypeVar, Union

from typing import Any, Callable, List, Sequence, Tuple, TypeVar, Union, DefaultDict

T = TypeVar("T")