gpt-oss sft megatron does not support sequence packing

**Describe the bug**

When set `sequence_packing=true` and use Megatron backend for fine-tuning gpt-oss. The debug info says no attention backend available.
If `sequence_packing=False`, only `UnfusedAttention` is available, `FlashAttention` disabled for softmax_type = learnable, `FusedAttention` disabled as no backend supports the provided input.

This is different from the fine-tuning in megatron-bridge, which support `FusedAttention` with learnable softmax.

Update: The different backends enabled are due to different versions of cuDNN in docker image. Latest nemo-rl nano image has cuDNN version = 91002, but in nemo image cuDNN version is 91310.

**Steps/Code to reproduce bug**

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A helpful guide on on how to craft a minimal bug report http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports.

**Expected behavior**

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**Additional context**

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