Add pip install update to resolve import error in transformers notebook (#8616)

* Add pip install update to resolve import error

Add pip install upgrade tensorflow-gpu to remove error below:
```
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-2-094fadb93f3f> in <module>()
      1 import torch
----> 2 from transformers import AutoModel, AutoTokenizer, BertTokenizer
      3 
      4 torch.set_grad_enabled(False)

4 frames
/usr/local/lib/python3.6/dist-packages/transformers/__init__.py in <module>()
    133 
    134 # Pipelines
--> 135 from .pipelines import (
    136     Conversation,
    137     ConversationalPipeline,

/usr/local/lib/python3.6/dist-packages/transformers/pipelines.py in <module>()
     46     import tensorflow as tf
     47 
---> 48     from .modeling_tf_auto import (
     49         TF_MODEL_FOR_QUESTION_ANSWERING_MAPPING,
     50         TF_MODEL_FOR_SEQ_TO_SEQ_CAUSAL_LM_MAPPING,

/usr/local/lib/python3.6/dist-packages/transformers/modeling_tf_auto.py in <module>()
     49 from .configuration_utils import PretrainedConfig
     50 from .file_utils import add_start_docstrings
---> 51 from .modeling_tf_albert import (
     52     TFAlbertForMaskedLM,
     53     TFAlbertForMultipleChoice,

/usr/local/lib/python3.6/dist-packages/transformers/modeling_tf_albert.py in <module>()
     22 import tensorflow as tf
     23 
---> 24 from .activations_tf import get_tf_activation
     25 from .configuration_albert import AlbertConfig
     26 from .file_utils import (

/usr/local/lib/python3.6/dist-packages/transformers/activations_tf.py in <module>()
     52     "gelu": tf.keras.layers.Activation(gelu),
     53     "relu": tf.keras.activations.relu,
---> 54     "swish": tf.keras.activations.swish,
     55     "silu": tf.keras.activations.swish,
     56     "gelu_new": tf.keras.layers.Activation(gelu_new),

AttributeError: module 'tensorflow_core.python.keras.api._v2.keras.activations' has no attribute 'swish'
```
I have tried running the colab after this change and it seems to work fine (all the cells run with no errors).

* Update notebooks/02-transformers.ipynb

only need to upgrade tensorflow, not tensorflow-gpu.
Co-authored-by: Lysandre Debut <lysandre@huggingface.co>
Co-authored-by: Lysandre Debut <lysandre@huggingface.co>

notebooks/02-transformers.ipynb

@@ -87,7 +87,7 @@
    "outputs": [],
    "source": [
     "!pip install transformers\n",
-    "!pip install tensorflow==2.1.0"
+    "!pip install --upgrade tensorflow"
    ]
   },
   {