Fix checkpoint export errors for the Dream model

[nitsanluke]

✨ Description

PR fixes existing issue in exporting a diffusion model.

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📝 Changes

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📊 Performance Impact Details

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[jlamypoirier]

Just a heads up, #264 added a new bug (@bigximik) TypeError: Can't instantiate abstract class HybridSSMModel without an implementation for abstract method 'get_tflops'. We previously used the trainer inheritance for it, but the HybridSSMModel doesn't inherit from GPT. Simplest fix would be to make it inherit, the formula will be wrong but at least the crash will be gone. (edit: fixed in #310)

[jlamypoirier]

One minor comment, otherwise LGTM