[TTS] Scale sampler steps by number of devices
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# What does this PR do ?

The weighted random sampler is configured to produce a fixed number of training samples during each epoch. This means if you use multiple GPUs, then the number of steps in each epoch is scaled down by the number of GPUs. This change multiplies the number of samples by the world size to keep the step count independent of number of GPUs used.

**Collection**: [TTS]

# Changelog 
- Scale number of samples per epoch by world size when using weighted random sampler.

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**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation
