Remove test df from alstm code

[chenditc]

Remove test df from alstm, it's not used.

Description

Remove test df from alstm, it's not used.

Motivation and Context

To prevent information leak, model should still be able to train even when test segment is not defined.

How Has This Been Tested?

• Pass the test by running: pytest qlib/tests/test_all_pipeline.py under upper directory of qlib.
• If you are adding a new feature, test on your own test scripts.

Screenshots of Test Results (if appropriate):

1. Pipeline test:
2. Your own tests:
   qrun completed with a config which has no test segment sepcified:

qlib_init:
    provider_uri: "~/.qlib/qlib_data/cn_data"
    region: cn
market: &market csi300
benchmark: &benchmark SH000300
data_handler_config: &data_handler_config
    start_time: 2005-05-01
    end_time: 2022-08-01
    fit_start_time: &fit_start_time 2005-05-01
    fit_end_time: &fit_end_time 2018-12-31
    instruments: *market
    infer_processors:
        - class: RobustZScoreNorm
          kwargs:
              fields_group: feature
              clip_outlier: true
        - class: Fillna
          kwargs:
              fields_group: feature
    learn_processors:
        - class: DropnaLabel
        - class: CSRankNorm
          kwargs:
              fields_group: label
task:
    model:
        class: ALSTM
        module_path: qlib.contrib.model.pytorch_alstm
        kwargs:
            d_feat: 6
            hidden_size: 64
            num_layers: 2
            dropout: 0.0
            n_epochs: 200
            lr: 1.0e-3
            early_stop: 20
            batch_size: 800
            metric: loss
            loss: mse
            GPU: 0
            rnn_type: GRU
    dataset:
        class: DatasetH
        module_path: qlib.data.dataset
        kwargs:
            handler:
                class: Alpha360
                module_path: qlib.contrib.data.handler
                kwargs: *data_handler_config
            segments:
                train: [*fit_start_time, *fit_end_time]
                valid: [2019-01-01, 2022-08-01]

Types of changes

• Fix bugs
• Add new feature
• Update documentation