[Fix] gpu collection during evaluation

[edkair]

results variable was not created when collect_device='gpu', causing an error.

[CLAassistant]

All committers have signed the CLA.

[zhouzaida]

Hi @edkair , thanks for you contribution.

[OpenMMLab-Assistant-004]

Hi @edkair,

We'd like to express our appreciation for your valuable contributions to the mmengine. Your efforts have significantly aided in enhancing the project's quality.
It is our pleasure to invite you to join our community thorugh Discord_Special Interest Group (SIG) channel. This is a great place to share your experiences, discuss ideas, and connect with other like-minded people. To become a part of the SIG channel, send a message to the moderator, OpenMMLab, briefly introduce yourself and mention your open-source contributions in the #introductions channel. Our team will gladly facilitate your entry. We eagerly await your presence. Please follow this link to join us: ​https://discord.gg/UjgXkPWNqA.

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Thanks again for your awesome contribution, and we're excited to have you as part of our community!