Add history_ attribute to DNN models for training loss tracking

[jiyanshuj]

Fixes #636

• Add self.history_ dict ({'loss': [...]}) to BaseDeepLearningDetector and all standalone DNN-based detectors (AutoEncoder, VAE, ALAD, AnoGAN, DeepSVDD, DevNet, LUNAR, MO_GAAL, SO_GAAL, TS-LSTM, TS-AnomalyTransformer, AE1SVM)
• Update docstrings to document the new attribute
• Add unit tests verifying history_['loss'] is populated per epoch
• No changes to existing verbose printing behavior, except DevNet's console log now prints the mean epoch loss instead of the last-batch loss (per review feedback)

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[Copilot]

Pull request overview

Adds a history_ attribute to deep-learning-based detectors so callers can retrieve training loss information after fit() (addressing issue #636), and extends the test suite to assert the history is populated.

Changes:

• Track and expose per-epoch training loss via history_ across BaseDeepLearningDetector-based models and several standalone DNN detectors.
• Update model docstrings and CHANGES.txt to document the new history_ attribute.
• Add/extend unit tests to validate history_ is present and populated after fitting.

Reviewed changes

Copilot reviewed 21 out of 21 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
pyod/models/base_dl.py	Initializes/resets and appends to history_['loss'] during training in the shared deep-learning base class.
pyod/models/auto_encoder.py	Documents inherited history_ for AutoEncoder.
pyod/models/vae.py	Documents inherited history_ for VAE.
pyod/models/deep_svdd.py	Adds history_ initialization and records per-epoch loss.
pyod/models/ae1svm.py	Adds history_ and records per-epoch mean loss (also reuses that value for best-loss tracking/printing).
pyod/models/devnet.py	Adds history_ and records per-epoch mean loss (also changes printed loss value).
pyod/models/lunar.py	Adds history_ and records loss each epoch.
pyod/models/ts_lstm.py	Adds history_ and records mean batch loss per epoch during training.
pyod/models/ts_anomaly_transformer.py	Adds history_ and records per-epoch mean minimize-phase loss.
pyod/models/anogan.py	Adds history_ mapping to GAN loss histories; reinitializes on fit().
pyod/models/alad.py	Adds history_ mapping to GAN loss histories during model build/fit.
pyod/models/so_gaal.py	Exposes existing train_history via history_ and documents it.
pyod/models/mo_gaal.py	Exposes existing train_history via history_ and documents it.
pyod/test/test_auto_encoder.py	Adds assertions that history_['loss'] exists and has epoch_num entries.
pyod/test/test_vae.py	Adds assertions that history_['loss'] exists and has epoch_num entries.
pyod/test/test_deepsvdd.py	Adds assertions that history_['loss'] exists and has epochs entries.
pyod/test/test_anogan.py	Adds assertions for GAN loss keys in history_.
pyod/test/test_alad.py	Adds assertions for GAN loss keys in history_.
pyod/test/test_so_gaal.py	Adds assertions for GAN loss keys in history_.
pyod/test/test_mo_gaal.py	Adds assertions for GAN loss keys in history_.
CHANGES.txt	Adds release-note entry for the new history_ attribute.

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