AttributeError: 'DatasetDict' object has no attribute 'features'
## Describe the bug
Receiving the error when trying to check for Dataset features

## Steps to reproduce the bug
from datasets import Dataset
dataset = Dataset.from_pandas(df[['id', 'words', 'bboxes', 'ner_tags', 'image_path']])

dataset.features

## Expected results
A clear and concise description of the expected results.

## Actual results
Getting the following errror

AttributeError: 'DatasetDict' object has no attribute 'features'

## Environment info
Copy-and-paste the text below in your GitHub issue.

- `datasets` version: 1.18.4
- Platform: Linux-4.14.252-131.483.amzn1.x86_64-x86_64-with-glibc2.9
- Python version: 3.6.13
- PyArrow version: 6.0.1
