Fix bug in RNNT Joint WER calculation for fused batch
jenkins
# What does this PR do ?

Fix metric calculation in the case fused batch size > 1 during training / validation calculation.

Previously, the RNNT metric was stateful while the CTC one was not ([r1.22.0](https://github.com/NVIDIA/NeMo/blob/r1.22.0/nemo/collections/asr/metrics/rnnt_wer_bpe.py#L419-L420), [r1.23.0](https://github.com/NVIDIA/NeMo/blob/r1.23.0/nemo/collections/asr/metrics/wer.py#L333))

Therefore this calculation in the RNNT joint for fused operation worked properly. However with the unification of metrics in r1.23.0, a bug was introduced where only the last sub-batch of metrics calculates the scores and does not accumulate.

**Collection**: [ASR]

# Changelog 
- Preserve the list of metric outputs from each sub-batch
- take average of batch wer, compute running sum of numerator and denominator


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