def fit(self, X, y=None, eval_data=None, eval_metric='acc',
            epoch_end_callback=None, batch_end_callback=None, kvstore='local', logger=None,
            work_load_list=None, monitor=None, eval_end_callback=LogValidationMetricsCallback(),
            eval_batch_end_callback=None):

训练集 Training data.——X: 格式：DataIter, or numpy.ndarray/NDArray
If X is a DataIter, the name or (if name not available) the position of its outputs should match the corresponding variable names defined in the symbolic graph.

训练集标签 Training set label.——y : 格式：numpy.ndarray/NDArray, optional
If Xis numpy.ndarray or NDArray, y is required to be set.
While y can be 1D or 2D (with 2nd dimension as 1), its first dimension must be the same as X,
i.e. the number of data points and labels should be equal.

验证集——eval_data : 格式：DataIter or numpy.ndarray/list/NDArray pair
If eval_data is numpy.ndarray/list/NDArray pair, it should be (valid_data, valid_label).

验证标准The evaluation metric. ——eval_metric 格式 : metric.EvalMetric or str or callable
This could be the name of evaluation metric or a custom evaluation function that returns statistics based on a minibatch.

回调函数（epoch结束时执行）——** epoch_end_callback** 格式: callable(epoch, symbol, arg_params, aux_states)
可以用来每个epoch保存一下模型（checkpoint）

回调函数（batch结束时执行）——batch_end_callback 格式: callable(epoch)
A callback that is invoked at end of each batch for purposes of printing.

将参数存储到哪
kvstore: KVStore or str, optional
The KVStore or a string kvstore type: 'local', 'dist_sync', 'dist_async'
In default uses 'local', often no need to change for single machiine.
KVStore behavior
- 'local', multi-devices on a single machine, will automatically choose best type.
- 'dist_sync', multiple machines communicating via BSP.
- 'dist_async', multiple machines with asynchronous communication.

是否打印日志
logger : logging logger, optional
When not specified, default logger will be used.

    work_load_list : float or int, optional
        The list of work load for different devices,
        in the same order as `ctx`.
    Note

    """

        data = self._init_iter(X, y, is_train=True)
        eval_data = self._init_eval_iter(eval_data)
        if self.sym_gen:
            self.symbol = self.sym_gen(data.default_bucket_key) # pylint: disable=no-member
            self._check_arguments()
        self.kwargs["sym"] = self.symbol
        arg_names, param_names, aux_names = \
                self._init_params(data.provide_data+data.provide_label)

        # setup metric
        if not isinstance(eval_metric, metric.EvalMetric):
            eval_metric = metric.create(eval_metric)
        # create kvstore
        (kvstore, update_on_kvstore) = _create_kvstore(
            kvstore, len(self.ctx), self.arg_params)
        param_idx2name = {}
        if update_on_kvstore:
            param_idx2name.update(enumerate(param_names))
        else:
            for i, n in enumerate(param_names):
                for k in range(len(self.ctx)):
                    param_idx2name[i*len(self.ctx)+k] = n
        self.kwargs["param_idx2name"] = param_idx2name

        # init optmizer
        if isinstance(self.optimizer, str):
            batch_size = data.batch_size
            if kvstore and 'dist' in kvstore.type and '_async' not in kvstore.type:
                batch_size *= kvstore.num_workers
            optimizer = opt.create(self.optimizer,
                                  rescale_grad=(1.0/batch_size),
                                  **(self.kwargs))
        elif isinstance(self.optimizer, opt.Optimizer):
            if not optimizer.idx2name:
                optimizer.idx2name = param_idx2name.copy()
            optimizer = self.optimizer

        # do training
        _train_multi_device(self.symbol, self.ctx, arg_names, param_names, aux_names,
                            self.arg_params, self.aux_params,
                            begin_epoch=self.begin_epoch, end_epoch=self.num_epoch,
                            epoch_size=self.epoch_size,
                            optimizer=optimizer,
                            train_data=data, eval_data=eval_data,
                            eval_metric=eval_metric,
                            epoch_end_callback=epoch_end_callback,
                            batch_end_callback=batch_end_callback,
                            kvstore=kvstore, update_on_kvstore=update_on_kvstore,
                            logger=logger, work_load_list=work_load_list, monitor=monitor,
                            eval_end_callback=eval_end_callback,
                            eval_batch_end_callback=eval_batch_end_callback,
                            sym_gen=self.sym_gen)