Fix RNN-T loss memory usage
State: closed | Pull request: yes
Labels: name: ASR

name: Run CICD

# What does this PR do ?

Fixes memory usage for Numba-based implementation of RNN-T and Multi-blank Transducer losses.
The current implementation requires 3x memory compared to the size of logits (logits, gradient, extra memory = size of logits). This PR fixes memory usage to the minimal possible 2x (logits, gradient). 

It looks like assigning tensors directly to `ctx` instead of saving them through `save_for_backward` breaks PyTorch logic, and it copies the gradient tensor (which results in extra memory usage).

TDT loss was not affected by this issue (I'm unsure why, but it requires contiguous tensor for labels-related logits).

**Before (main):**
- Peak memory before loss: 2.09 GB
- Peak memory after loss: **6.27 GB**

**After (this PR):**
- Peak memory before loss: 2.09 GB
- Peak memory after loss: **4.18 GB**

Code to check memory usage (size of tensors besides logits is negligible compared to logits):
```python
import torch
from nemo.collections.asr.parts.numba.rnnt_loss import RNNTLossNumba
device = torch.device("cuda")

loss = RNNTLossNumba(blank=1023, reduction='none')
batch_size = 32
logits = torch.rand([batch_size, 188, 90 + 1, 1024], device=device, dtype=torch.float32, requires_grad=True)
encoder_lengths = torch.full([batch_size], fill_value=188, device=device, dtype=torch.long)
label_lengths = torch.full([batch_size], fill_value=90, device=device, dtype=torch.long)
labels = torch.randint(0, 1022, size=[batch_size, 90], dtype=torch.long, device=device)
print(f"Peak memory before loss: {torch.cuda.max_memory_allocated() / (2 ** 30):.2f} GB")

loss_value = loss(acts=logits, act_lens=encoder_lengths, labels=labels, label_lens=label_lengths)
loss_value.sum().backward()
print(f"Peak memory after loss: {torch.cuda.max_memory_allocated() / (2 ** 30):.2f} GB")
```

**Collection**: [ASR]

# Changelog 
- In transducer-related functions that extend autograd, save tensors using `ctx.save_for_backward(...)` instead of directly assigning tensors [according to PyTorch documentation](https://pytorch.org/docs/stable/notes/extending.html#extending-autograd).

# Usage
* You can potentially add a usage example below

```python
# Add a code snippet demonstrating how to use this 
```

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[🤖]: Hi @artbataev 👋,

We wanted to let you know that a [CICD pipeline](https://github.com/NVIDIA/NeMo/actions/runs/11667864007) for this PR just finished successfully

So it might be time to merge this PR or get some approvals

I'm just a bot so I'll leave it you what to do next.

//cc @pablo-garay @ko3n1g