fix(wandb): use is_wandb_available function in callbacks

import tensorflow as tf

import sys

from deepchem.models.keras_model import is_wandb_available

if is_wandb_available():

  import wandb

class ValidationCallback(object):

  """Performs validation while training a KerasModel.

except (ImportError, AttributeError):

  _has_wandb = False

def is_wandb_available():

  return _has_wandb

class KerasModel(Model):

  """This is a DeepChem model implemented by a Keras model.

    self.tensorboard = tensorboard

    # W&B logging

    if wandb and not _has_wandb:

    if wandb and not is_wandb_available():

      logger.warning(

        "You set wandb to True but W&B is not installed. To use wandb logging, "

        "run `pip install wandb; wandb login` see https://docs.wandb.com/huggingface."

      )

    self.wandb = wandb and _has_wandb

    self.wandb = wandb and is_wandb_available()

    # Backwards compatibility

    if "tensorboard_log_frequency" in kwargs: